Emit otel metrics directly instead of through tracing

[rylev]

Alternative to #3619

Instead of doing some compile time magic, we instead get rid of our dependency on tracing for emitting metrics and instead emit the otel metrics directly.

This does require duplicating macros to specify types as as far as I can tell we can't infer the type param we need to pass to the opentelemetry::metrics types.

I also deleted some tests in crates/connection-semaphore/src/lib.rs where were testing that we were emitting specific metrics. This is harder to test with the new implementation, and I'm not convinced we actually got that much out of those tests.

[lann]

This does require duplicating macros to specify types as as far as I can tell we can't infer the type param we need to pass to the opentelemetry::metrics types.

This seems more robust than what tracing-opentelemetry does anyway: https://docs.rs/tracing-opentelemetry/latest/tracing_opentelemetry/struct.MetricsLayer.html#floating-point-numbers

[lann]

Minor quibbles. I think this is better than tracing.

[lann]

I suppose this should be updated to mention that it sets the global metrics provider. An LLM might read it...

[calebschoepp]

I like this approach a lot more