ddtrace/tracer: add source tag to spans_started health metric

[hannahkm]

What does this PR do?

Add a source tag to the existing datadog.tracer.spans_started metric.

Motivation

We want to know, in addition to when a span is started, where the span originated from. This could be a contrib or a manual implementation.

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: apm-rd/span-source-health-metric
Commit report: e026378
Test service: dd-trace-go

✅ 0 Failed, 5110 Passed, 67 Skipped, 2m 26.11s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-12-10 21:54:27

Comparing candidate commit 819e312 in PR branch apm-rd/span-source-health-metric with baseline commit 1d2c8f1 in branch main.

Found 0 performance improvements and 2 performance regressions! Performance is the same for 57 metrics, 0 unstable metrics.

scenario:BenchmarkHttpServeTrace-24

• 🟥 allocations [+4; +4] or [+5.063%; +5.063%]
• 🟥 execution_time [+552.417ns; +661.983ns] or [+3.459%; +4.145%]