GitLab Runner http endpoint should default to 9252

When using Prometheus, there is a list of reserved ports for each exporter. You can see that list here: https://github.com/prometheus/prometheus/wiki/Default-port-allocations

This reduces the chance of multiple exporters running on the same machine and having port conflicts. You can see we have reserved a number of ports for other GitLab services, for example gitaly and workhorse.

I have gone ahead and reserved 9252 for Runner, and it would be great if we defaulted to this port in the event no other port was specified.

@tmaczukin Accepting Merge Requests on this one?

@markglenfletcher Seems like a good one to me. Should be straight forward and minor change, just really about cleanliness.

@markglenfletcher @joshlambert Currently metrics server by default is not configured and not started. User need to set the metrics-server value in config.toml explicitly to start it.

Such change should be implemented:

• in documentation: suggesting to use port 9252, showing it in the examples and probably linking to https://github.com/prometheus/prometheus/wiki/Default-port-allocations,
• in code: commands.configOptionsWithMetricsServer.metricsServerAddress() should check if there is a port set explicitly in the address and if not, set it to 9252.

The change will be relatively easy to do and I think we can label it with Accepting Merge Requests. However let's schedule this to %9.3 to make this happen before next release :)

added Accepting Merge Requests ~60197 improvement metrics labels

changed milestone to %9.3

@tmaczukin I started working on this but was wondering now whether I should put the 9252 number directly in the metricsServerAddress() or is there any file where constants like these should be saved? I'm not familiar with the code base yet so I wasn't sure.

@Rukenshia Please add it here - https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/common/consts.go - and then use in metricsServerAddress().

mentioned in merge request !584 (merged)

closed via merge request !584 (merged)

added Documentation and removed ~60197 labels