opentelemetry tracer: add OTLP/HTTP exporter

[joaopgrassi]

Commit Message:
Add OTLP/HTTP exporter to OpenTelemetry Tracer

Co-authored-by: Alex Ellis ellisonjtk@gmail.com

Additional Description: This PR refactors the OpenTelemetry tracer, allowing it to export OTLP data via HTTP. It refactors the existing gRPC exporter into a general exporter, so the functionality can be shared between the gRPC and the new HTTP exporters. The HTTP exporter is relatively simple: it accesses the thread local cluster for the configured cluster, then sends the OTLP request.

Risk Level: Low (new, optional functionality for existing tracing extension)

Testing: Unit test and manual tests performed, exporting spans to a local OTel collector and a public/external back-end via OTLP/HTTP. Also performed tests exporting to a local OTel collector via gRPC to ensure the existing feature is unaffected.

Fixes #26352

Note: This is a continuation of #28454 as previously discussed and agreed with @AlexanderEllis and @htuch.

For posterity, here's how a "full" configuration looks like:

tracing:
  provider:
    name: envoy.tracers.opentelemetry
    typed_config:
      "@type": type.googleapis.com/envoy.config.trace.v3.OpenTelemetryConfig
      service_name: envoy-HTTP-exporter
      http_config:
        cluster_name: my_backend
        traces_path: "/api/otlp/v1/traces"
        hostname: "my-backend.com"
        headers:
          - key: "Authorization"
            value: "auth-token"
          - key: "x-custom-header"
            value: "something-custom"
        timeout: 5s

[repokitteh-read-only]

Hi @joaopgrassi, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #29207 was opened by joaopgrassi.

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 @adisuissa
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #29207 was opened by joaopgrassi.

see: more, trace.

[phlax]

@AlexanderEllis @htuch @adisuissa seems this is wating on further review/feedback

/wait-any

[htuch]

LGTM modulo open thread on header formatter.

[joaopgrassi]

/docs

@wbpcode @htuch I went with option 2 - modified the opentelemetry.proto docs to make it clear there's no formatting. All should be resolved and good to go now!

Link to the docs: https://storage.googleapis.com/envoy-pr/d57e0ea/docs/api-v3/config/trace/v3/opentelemetry.proto.html

[repokitteh-read-only]

Docs for this Pull Request will be rendered here:

https://storage.googleapis.com/envoy-pr/29207/docs/index.html

The docs are (re-)rendered each time the CI envoy-presubmit (precheck docs) job completes.

🐱

Caused by: a #29207 (comment) was created by @joaopgrassi.

see: more, trace.

[wbpcode]

Thanks. Some new comments are added.

[joaopgrassi]

@wbpcode tried addressing the "copy" of headers. Please let me know what you think! thank you 🙌

[wbpcode]

LGTM. Thanks.

[wbpcode]

LGTM. Thanks.