Add Alpine Docker image

Shrink binary and image

treat string "err" as failure and return zero for gauge

bring back some accidentally dropped commits

fix query in dashboard

Cleanup some metrics

Remove `mem_fragmentation_ratio`:
* Comes from `redis_memory_used_rss_bytes / redis_memory_used_bytes`.
* Metric only provides 2 digits of accuracy compared to calculation.

Add allocator metrics.

Cleanup Prometheus naming of commands metrics names to avoid confusing
references to being a "Summary" metric, it's not.

Remove `total_system_memory`, this duplicates node_exporter metrics.

Signed-off-by: Ben Kochie <superq@gmail.com>

feat: add aof metrics

Update to tests to use Redis5, Golang 1.11 for tests and build

Add slowlog_last_id metric.

use codecov

* Avoid using KEYS.

* Check single keys, or patterns with `SCAN`

Continues the work that @Tinche did in #149.

* Add an arg, `check-single-keys` to get size and value with `TYPE`
* Modify the behavior of `check-keys`
  * Get keys from globs with `SCAN` rather than `KEYS`
  * If a key doesn't have any glob characters in it, just add it to the
    list of things to check with `TYPE`

I added an argument rather than decide `SCAN` vs `TYPE` based on the
presence of glob characters because, technically, redis key names can
contain glob characters, according to https://redis.io/topics/data-types-intro:

    Redis keys are binary safe, this means that you can use any binary
    sequence as a key, from a string like "foo" to the content of a JPEG
    file. The empty string is also a valid key.

The only way to be able to check for key names with glob characters in
them is by being able to explicitly specify keys that you don't want to
glob for.

* add empty help text to be compatible with prometheus >= 2.3.1

* Properly decode uptime from redis
* Add uptime_in_seconds & start_time_seconds to expected metrics

Add alternative credentials claim for host.

Add start_time_seconds