enhancement(logplex source): Add instrumentation

[ktff]

Closes #3202

Open questions

• If we add HerokuLogplexEventReceived we would count one event twice since enhancement(observability): Add internal events for http source #3264 added HTTPEventsReceived to a shared code that logplex source uses.
  To fix this, I propose moving emit!(HTTPEventsReceived) from here https://github.com/timberio/vector/blob/547c8ad6ac6ad4fa21dbf3a70f6b18570704bed6/src/sources/util/http.rs#L90-L93 to here https://github.com/timberio/vector/blob/547c8ad6ac6ad4fa21dbf3a70f6b18570704bed6/src/sources/http.rs#L58 And also removing this label https://github.com/timberio/vector/blob/547c8ad6ac6ad4fa21dbf3a70f6b18570704bed6/src/internal_events/http.rs#L51 so to make it clear these errors are aggregated from multiple types of sources.
  ref. Document internal instrumentation rules #3247 (comment) (Edit. Include internal trace context with internal metrics #2660 will solve this)

[binarylogic]

Thanks @ktff. I assume #2660 will fix this issue when it's complete?

[ktff]

I assume #2660 will fix this issue when it's complete?

@binarylogic Yes, but trace context would need to be main source of truth. So it's fine to leave this as it is?

[binarylogic]

For now it should be fine. Once #2660 is fine we’ll come back and update.