netstat: return nothing when /proc/net/snmp6 not found

* Correct buffer_bytes > INT_MAX on BSD/amd64.

The sysctl vfs.bufspace returns either an int or a long, depending on
the value.  Large values of vfs.bufspace will result in error messages
like:

  couldn't get meminfo: cannot allocate memory

This will detect the returned data type, and cast appropriately.

* Added explicit length checks per feedback.

* Flatten Value() to make it easier to read.

* Simplify per feedback.

* Fix style.

* Doc updates.

xfs: expose correct fields, fix metric names

* Remove unnecessary select statement

* Remove unnecessary if-statement

When there are no SMART compatible devices (Raspberry Pi for example) an
error is returned, but the return code is still 0.

`# scan_smart_devices: glob(3) aborted matching pattern /dev/discs/disc*`

* Remove unused `disks` variable.
* Filter for only valid `/dev` devices.

* Add `collect[]` parameter

* Add TODo comment about staticcheck ignored

* Restore promhttp.HandlerOpts

* Log a warning and return HTTP error instead of failing

* Check collector existence and status, cleanups

* Fix warnings and error messages

* Don't panic, return error if collector registration failed

* Update README

* Add link to Golang
* Add note about RHEL/CentOS build dep.

The default DOCKER_IMAGE_TAG setup fails when running in circle,
override with the CIRCLE_TAG.

* Release v0.15.0

* Bump version.
* Update CHANGELOG.

* Update to Go 1.9 in circle.yml

* Update vendor github.com/coreos/go-systemd/dbus@v15

* Update vendor github.com/ema/qdisc

* Update vendor github.com/godbus/dbus

* Update vendor github.com/golang/protobuf/proto

* Update vendor github.com/lufia/iostat

* Update vendor github.com/matttproud/golang_protobuf_extensions/pbutil@v1.0.0

* Update vendor github.com/prometheus/client_golang/...

* Update vendor github.com/prometheus/common/...

* Update vendor github.com/prometheus/procfs/...

* Update vendor github.com/sirupsen/logrus@v1.0.3

Adds vendor golang.org/x/crypto

* Update vendor golang.org/x/net/...

* Update vendor golang.org/x/sys/...

* Update end to end output.

Only enable race detector when GOHOSTARCH is amd64.

* Add metric for pending upgrades.
* Add metric for pending reboot required.

The github.com/beevik/ntp package was recently updated with some
API changes that broke node_exporter. This commit fetches the
latest version of the ntp package and brings node_exporter in
line with the latest API.

This enables native builds to still run the test and all targets without
problems on say 386.

Build failure on Buildkite build 85, prevents enabling native FreeBSD
386 builds.

* Move NodeCollector into package collector

* Refactor collector enabling

* Update README with new collector enabled flags

* Fix out-of-date inline flag reference syntax

* Use new flags in end-to-end tests

* Add flag to disable all default collectors

* Track if a flag has been set explicitly

* Add --collectors.disable-defaults to README

* Revert disable-defaults flag

* Shorten flags

* Fixup timex collector registration

* Fix end-to-end tests

* Change procfs and sysfs path flags

* Fix review comments

This collector is based on adjtimex(2) system call.  The collector returns
three values, status if time is synchronised, offset to remote reference,
and local clock frequency adjustment.

Values are taken from kernel time keeping data structures to avoid getting
involved how the synchronisation is implemented.  By that I mean one should
not care if time is update using ntpd, systemd.timesyncd, ptpd, and so on.
Since all time sync implementation will always end up telling to kernel what
is the status with time one can simply omit the software in between, and
look results of the syncing.  As a positive side effect this makes collector
very quick and conceptually specific, this does not monitor availability of
NTP server, or network in between, or dns resolution, and other unrelated
but necessary things.

Minimum set of values to keep eye on are the following three:

    The node_timex_sync_status tells if local clock is in sync with a remote
    clock.  Value is set to zero when synchronisation to a reliable server
    is lost, or a time sync software is misconfigured.

    The node_timex_offset_seconds tells how much local clock is off when
    compared to reference.  In case of multiple time references this value
    is outcome of RFC 5905 adjustment algorithm.  Ideally offset should be
    close to zero, and it depends about use case how large value is
    acceptable.  For example a typical web server is probably fine if offset
    is about 0.1 or less, but that would not be good enough for mobile phone
    base station operator.

    The node_timex_freq tells amount of adjustment to local clock tick
    frequency.  For example if offset is one second and growing the local
    clock will need instruction to tick quicker.  Number value itself is not
    very important, and occasional small adjustments are fine.  When
    frequency is unusually in stable one can assume quality of time stamps
    will not be accurate to very far in sub second range.  Obviously
    explaining why local clock frequency behaves like a passenger in roller
    coaster is different matter.  Explanations can vary from system load, to
    environmental issues such as a machine being physically too hot.

Rest of the measurements can help when debugging.  If you run a clock server
do probably want to collect and keep track of everything.

Pull-request: https://github.com/prometheus/node_exporter/pull/664

* Add metrics from SNTPv4 packet to ntp collector & add ntpd sanity check

1. Checking local clock against remote NTP daemon is bad idea, local
ntpd acting as a  client should do it better and avoid excessive load on
remote NTP server so the collector is refactored to query local NTP
server.

2. Checking local clock against remote one does not check local ntpd
itself. Local ntpd may be down or out of sync due to network issues, but
clock will be OK.

3. Checking NTP server using sanity of it's response is tricky and
depends on ntpd implementation, that's why common `node_ntp_sanity`
variable is exported.

* `govendor add golang.org/x/net/ipv4`, it is dependency of github.com/beevik/ntp

* Update github.com/beevik/ntp to include boring SNTP fix

* Use variable name from RFC5905

* ntp: move code to make export of raw metrics more explicit

* Move NTP math to `github.com/beevik/ntp`

* Make `golint` happy

* Add some brief docs explaining `ntp` #655 and `timex` #664 modules

* ntp: drop XXX comment that got its decision

* ntp: add `_seconds` suffix to relevant metrics

* Better `node_ntp_leap` comment

* s/node_ntp_reftime/node_ntp_reference_timestamp_seconds/ as requested by @discordianfish

* Extract subsystem name to const as suggested by @superq

* cpu: Metric 'package_throttles_total' is per package.

'package_throttles_total' is per package, not per cpu. This also reduces
the total number of cpu time series a lot (esp for multi core cpus).

* cpu: Better handling of a cpulist edge-case.

* cpu: Extract the package number from the directory name.

Do not rely on the range index.

* cpu: Add package_throttle_count for node0 cpu1

This file must be ignored by the cpu collector.

* Always try to return smartmon_device_info metric

Sometimes the 'model family' field is not returned by `smartctl' because
a disk is not in the disk database for the version of smartmontools
installed on the system.

In those cases, the device model and serial number is still returned (at
least as far as I have observed.

Re-work the logic to prefer the 'vendor' field first, and if not
present, always output a `smartmon_device_info` metric even if some
labels have empty values.

On the box I'm testing this on, where previously no metric was returned,
it now returns:

    # HELP smartmon_device_info SMART metric device_info
    # TYPE smartmon_device_info gauge
    smartmon_device_info{disk="/dev/sda",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sdb",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sdc",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sdd",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sde",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sdf",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1

* Add trailing newline

Because POSIX:
https://stackoverflow.com/a/729795

Add buildkite status badge.

This doesn't work on at least FreeBSD and Darwin. It does work on Linux,
only try it there.

Automatically cross-test 32bit based on GOARCH

Use int64 throughout the ZFS collector.

Try to determine the corresponding 32bit architecture from the current
GOARCH and run the tests under that architecture. This only works on a
GOOS/GOARCH that can execute binaries for the smaller architecture, such
as running linux/386 binaries under linux/amd64.

I tested that this works under linux/amd64 and darwin/amd64, the rest of
the architectures is guesswork.

While we still only run regular tests on Intel/Linux architectures, this
covers general integer overflow issues like #629.

This avoids issues with integer overflows on 32-bit architectures. The
Prometheus data format is float64, so regardless of the architecture we
should handle large numbers.

Fixes #629.

Fix path and timing issues with integration tests.

Ignore the permission denined error when the wifi collector has no
permission to read metrics.

* Add additional field to github issue template.

Request the command line flags to the exporter.

* Update version flag for kingpin.

* Add dockerfile for ppc64le and related changes

* Pass the fill file as DOCKEFILE

* Add the dockerfile name to build msg