Context change interceptor

[bogdandrutu]

This feature was requested initially in #295 and moved here because all other points are addressed.

One of the goal is to support log correlation, but at one point @kbrockhoff mentioned that what OpenCensus implemented https://github.com/census-instrumentation/opencensus-java/tree/master/contrib/log_correlation/log4j2 is not good, can you please clarify?

[bogdandrutu]

@tylerbenson you asked for a prototype on how to build a real interceptor for Context changes, see #923

[bogdandrutu]

Ping @kbrockhoff

[carlosalberto]

Closing due to lack of activity. @kbrockhoff Please re-open if/as needed ;)

[trask]

@carlosalberto I think this is important, can we re-open?

The only way to create an OpenTelemetry scope listener currently is by creating a class with a very special name io.grpc.override.ContextStorageOverride. This seems like a very odd public API surface that we are exposing, and what happens if there is already one of these classes on the classpath?

Scope listeners are important for logging MDC integrations (to populate spanId and traceId into logging MDC).

[carlosalberto]

Marking this initially as after-ga. @trask let me know if you think we ought to have this for GA instead.

[trask]

The main use case we need this for is logging MDC. Since logging is out of scope for GA, I think this is ok, at least from instrumentation repo perspective.