Exporterhelper: address timeout misalignment w/ context deadline

[jmacd]

Description

Retry sender: if the context deadline will expire before a configured attempt to retry, abort the request. Otherwise, the retry sender will delay until the context timeout happens, which is a predictable waste of resources. Better to return immediately. As for the aborted status code, I could be convinced instead to stop retrying and return the final-retry-attempt's error.

Timeout sender: if the context deadline will expire before the full configured timeout, present two optional new behaviors in addition to the current behavior. The three are:

• sustain: existing behavior, let a short timeout pass through to the exporter
• abort: new behavior, do not send a request if the timeout expires after the context deadline
• ignore: new behavior, send a request with the configured timeout, ignore the context deadline.

Link to tracking issue

Fixes #11183.

Testing

TODO: This is draft for discussion purposes. I have not added or modified existing tests, and I'm aware of stylistic requirements that are not met (e.g., the use of bare string values, vs. use of a constrained value such as configcompression.Type).

Documentation

WIP: DRAFT.

[jmacd]

@bogdandrutu Thank you. I will work the rest of this PR and add tests.