Add runtime metrics support

[brnhffmnn]

This adds a metricproducer.Producer implementation under /plugin/ which enables the collection of runtime metrics.

Fixes #784

[rghetia]

Thanks for your contribution. See comments inline.

[rghetia]

Instead of creating producer and then require the user to register it, may be it can also register it.

func Enable(options RunMetricsOptions) error {
    // Based on options create and register cpu, mem  or both producers.
}
func Disable() {
    // find producer associated with the process and delete it from registry.
}

Restrict to a single producer per process.
After above changes there is no need to export Producer.

[rghetia]

This should be renamed to RunMetricOptions.

[rghetia]

nit: rename 'c' to 'p'.

[rghetia]

name should follow the convention used in opentelemetry-service

[brnhffmnn]

Could you please check if all names are ok with my last change.

[rghetia]

the names look fine. I didn't see cpu_seconds similar to one in vmmetricreceiver

[brnhffmnn]

CPU time is handled by prometheus/procfs in vmmetricsreceiver. I focused on what is available out-of-the-box.

[dprotaso]

Any thoughts on adding GC metrics?

ie. https://golang.org/pkg/runtime/debug/#GCStats

The Prometheus GoCollector exports it this way
https://github.com/prometheus/client_golang/blob/master/prometheus/go_collector.go#L319-L328

[rghetia]

There is a race condition here. Without any mutex there is a possibility of adding two producers.
Maybe you can create a singleton with sync.Once and with mutex delete/add producer when it is enabled/disabled.
You can also create all entries once and simply based on boolean (memory/cpu) read those metrics.

[brnhffmnn]

I wasn't able to see a real data race, but it was inconsistent indeed. I adjusted it, so that there can only be one producer registered. But if you would enable/disable concurrently, the last one wins (obviously).

I didn't go with a singleton, so the producer instance can remain immutable and doesn't need to care about synchronizing on the options or anything like that.