[Feature]: Merge user-event and ETW metrics exporter into single metrics exporter crate

[mattbodd]

Related Problems?

No response

What component are you working with?

opentelemetry-user-events-metrics

Describe the solution you'd like:

Today, we maintain two separate crates for user-event and ETW based exporters. Much of the implementation is the same and implementations significantly diverge at the point when platform specific APIs are used to transmit serialized metrics. The PushMetricsExporter implementations of the opentelemetry-user-event-metrics and opentelemetry-etw-metrics could be merged into a single implementation that internally invokes platform specific logic.

I am creating this issue as a point of conversation. Following up on a comment from @lalitb on #43.

Considered Alternatives

No response

Additional Context

No response

[psandana]

I'm 100% agree on this. We should also do the same for Logs, now that support is starting for ETW.