Differentiate Logs from Traces for tracing-opentelemetry

[hdost]

Originally brought up in #1085
Could apply to other libraries as well.
Example use case: Datadog only parses LogRecords not SpanEvents.

• Test methods for filtering
• Benchmark methods.

Tracing crate already has ability to emit Logs Always (need to explore - https://docs.rs/tracing/latest/tracing/#emitting-log-records)

You shouldn't enable this feature for opentelemetry-rust. it's meant for libraries like hyper or tokio to smoothly transition to tracing in a minor semver release, allowing users to not immediately lose logging when then update their dependencies.

How do we user decide if this should go as span_event or log? [...] We need to investigate/explore this more. (there may be "Filter" that can be leverages Metadata)

Using dynamic_filter_fn is an option, but be sure to benchmark this since it might be a little too costly. We can always improve it on the tracing side of things though, especially since there is some low-hanging, performance-related fruit in tracing-subscriber that we've been meaning to improve. Y'all saying that it's a little too slow would sufficient for me to justify the positive impact of me working on fixing it.

Originally posted by @davidbarsky in #1085 (comment)