contrib/envoyproxy: fix context propagation from envoy tracing

[e-n-0]

What does this PR do?

Fix the context propagation from the context injected by Envoy in its tracing capabilities.

The information about the context was set by envoy into the gRPC metadata. These metadata are now merged with the headers of the received request.

Motivation

The spans generated with an instrumented Envoy with APM didn't got correlated correctly with the spans generated by the service extensions.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

Unsure? Have a question? Request a review!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: flavien/envoy/fix-propagated-headers-apm-envoy
Commit report: da4ce3e
Test service: dd-trace-go

✅ 0 Failed, 5223 Passed, 72 Skipped, 2m 1.32s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-04 13:25:22

Comparing candidate commit bbac29e in PR branch flavien/envoy/fix-propagated-headers-apm-envoy with baseline commit 998add0 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 58 metrics, 1 unstable metrics.