Fix flaky jaeger test by fully closing managed channel to isolate tests

[jack-berg]

The JaegerRemoteSamplerGrpcNettyTest.unimplemented_error_server_response test fails 57% of the time: https://develocity.opentelemetry.io/scans/tests?search.timeZoneId=America%2FChicago&tests.container=io.opentelemetry.sdk.extension.trace.jaeger.sampler.JaegerRemoteSamplerGrpcNettyTest

Right now the theory is:

• Other tests configure tiny polling intervals (1ms), queueing up lots of requests to the shared armeria test server
• unimplemented_error_server_response starts, queues up a mock UNIMPLEMENTED error resposne, waits for a log to assert the jaeger sampler received and handled it correctly, but...
• the sampler never receives that error response because queued up requests from the jaeger samplers of other tests get served the response from the queue, leaving the jaeger sampler under test to get the default ok response

Solution is to try to isolate tests better by making sure requests from tests don't leak into each other. Accomplish this by a more thorough managed channel shutdown routine.

[jack-berg]

This is consistent with SpanExporter, MetricExporter, LogRecordExporter close implementation.

JaegerREmoteSamplerTest would be prone to the same test flake as JaegerRemoteSamplerGrpcNettyTest if it weren't for a bug in which the server called grpcErrors.peek() instead of poll(). This causes JaegerRemoteSamplerTest to avoid test flakes by ignoring the error queue and just returning the same error over and over.

I fixed the peek / poll but, but need to make sure shutdown is thorough to isolate test cases, else JaegerRemoteSamplerTest will start being flaky.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.27%. Comparing base (f998f3f) to head (1434a41).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8322   +/-   ##
=========================================
  Coverage     90.27%   90.27%           
  Complexity     7695     7695           
=========================================
  Files           850      850           
  Lines         23207    23207           
  Branches       2356     2356           
=========================================
  Hits          20951    20951           
  Misses         1531     1531           
  Partials        725      725           

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