Allow for setting metrics descriptions

[lsunsi]

Feature Request

Motivation

Opentelemetry (and the rust crate) allow for setting descriptions to metrics, as seen in https://docs.rs/opentelemetry/0.20.0/opentelemetry/metrics/struct.InstrumentBuilder.html#method.with_description for example. It's just a handy feature that generates both code documentation and complementary information to the metric.

It'd be nice to allow this possibility via tracing-opentelemetry, so users using this API won't be limited. The same goes for .with_unit as well!

Proposal

Maybe it could work with specially named properties like we have with "otel.name", maybe on metric events a "otel.description" property or something.

Alternatives

I didn't think of anything

[ymgyt]

I need this feature too.
My question in adding this feature is that the current api allows multiple metrics to be output from a single tracing event.

for example

tracing::info!(
  counter.request_count = 1,
  histgram.request_duration = 500,
  otel.description = "...",
);

How should we handle otel.description in such a case?

[lsunsi]

This is a great question, I haven't thought of it. Maybe it could use the name of the counter?

tracing::info!
  counter.request_count = 1,
  histgram.request_duration = 500,
  otel.request_count.description = "...",
  otel.request_duration.description = "..."
);

Just throwing out ideas