Add extension point to OpenTelemetry tracer for custom exporters.

[jakebennert]

Commit Message

Added support for custom exporters in the OpenTelemetry tracer.

In implementing this feature, I also changed some existing code in opentelemetry_tracer_impl.cc to make it more thread safe, such as moving the instantiation of the gRPC client factory out of the worker threads and into the main thread. This means that if the factory creation returns an error, it won't crash the worker thread, and thereby the whole Envoy proxy.

Additional Description

AI was used to generate the code and tests in this PR. I have reviewed – and fully understand – the code.

Risk Level

Low

Testing

I've added unit tests testing both the control plane and data plane code flows.

Docs Changes

None required

Release Notes

Added a changelog under new_features (area: tracing)

Platform Specific Features

None

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #46679 was opened by jakebennert.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @mattklein123
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #46679 was ready_for_review by jakebennert.

see: more, trace.