Offer operators to send custom metrics to OTel / Prometheus

[shadaj]

Right now, our demos such as Paxos use plain stdout logging to report throughput / latency numbers. We should offer operators / standard library functions for sending metrics to a centralized store, just like we provide library functions for throughput measurement.

[MingweiSamuel]

Basically, convert _counter() to use the new metrics system created by #2178/#2228

Old name:

Offer operators to send custom metrics to OTel / Prometheus