Library to support Telemetry.Metrics as OpenTelemetry metrics

[tsloughter]

Not ready because still using local changes in the deps but wanted to open it up for review while those changes are being merged and released.

[tsloughter]

Forgot I needed to make meter a per-metric option. Though that isn't actually going to work since the main purpose of this is supporting stuff like Phoenix which isn't going to add meter to the reporter opts of telemetry events...

So may just add a warning in the docs that Scope is going to be wrong for all your Telemetry.Metrics

[josevalim]

😍

[josevalim]

Suggested change

	@doc """
	@doc """
	Starts open telemetry metrics reporter process.
	Accepts the same options as `GenServer.start_link/2`.

[tsloughter]

The last line seems wrong? Or at least confusing. That'd make me think the options are ones for configuring the GenServer, stuff like hibernate, but its the options to be passed to init.

[josevalim]

Yes, you are right, sorry.

[AndrewDryga]

Can I help you guys somehow? Have to submit metrics to Google Metrics for alerting and, since our metrics are not mission critical, we can try to pilot this PR in production.

[tsloughter]

@AndrewDryga yes, definitely! Are you on slack? I hit a bug in metrics that has kept me from having time to spend on docs but could help you on slack to get going.