Add benchmark for TraceDigest generation

[marcotc]

Follow up to #3474

What does this PR do?

Adds a benchmark that measures the process of generating a TraceDigest from an active trace.
This happens whenever distributed tracing propagation happens.

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-02-23 21:40:29

Comparing candidate commit a72e37b in PR branch bm-digest with baseline commit 110997c in branch 2.0.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 metrics, 1 unstable metrics.