internal/telemetry: fix flake in TestTelemetryEnabled

[hannahkm]

What does this PR do?

Locks the mockClient before updating the values in Metrics. Also adds an improved error message for the previously failing line.

Closes APMLP-319

Motivation

There was a flake in which TestTelemetryEnabled/tracer_start did not find the correct init time due to a data race, causing random failures that impacts the health of the tracer. In the off chance that the flake reappears, the new error message should help with debugging more thoroughly.

Running the test ~2000 times on both a MacOS computer and in a Windows VM passed. Rerunning CI a few times also only gave successes.

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: hannahkm/fix-telemetry-flake
Commit report: c096a52
Test service: dd-trace-go

✅ 0 Failed, 5226 Passed, 73 Skipped, 1m 46.6s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-07 15:52:47

Comparing candidate commit 3bfb4f2 in PR branch hannahkm/fix-telemetry-flake with baseline commit 4a72a09 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 62 metrics, 0 unstable metrics.