Add OpenTelemetry metrics example

[rucek]

Adds a complete example of setting up OpenTelemetry metrics, using a gRPC exporter which sends the default metrics to an external collector.

[adamw]

cc @softberries, what do you think? :)

[kciesielski]

How about using Pekko instead of Akka? Or maybe even Netty-Future server? I think the latter is what we'd like to promote as our preferred Future backend, the example would also be a bit simpler.

[rucek]

How about using Pekko instead of Akka? Or maybe even Netty-Future server? I think the latter is what we'd like to promote as our preferred Future backend, the example would also be a bit simpler.

If this is the preferred one, I'd say why not. Then we can perhaps also update the other examples (of which this one is a copy) for consistency - WDYT?

[kciesielski]

That could be useful, in a separate PR. I guess we still want some Akka example(s) to show just how to use the Akka backend, but other examples could be rewritten to Pekko, wdyt @adamw?

[adamw]

That could be useful, in a separate PR. I guess we still want some Akka example(s) to show just how to use the Akka backend, but other examples could be rewritten to Pekko, wdyt @adamw?

Makes sense, either Pekko or maybe Netty-Future? Of course we should keep Akka for the example which showcase integration with Akka, but otherwise keeping the "noise" to a minimum is best to showcase a feature