ddtrace/tracer: initialize runtimeMetricsV2 with statsd "direct" client

[anatolebeuzon]

What does this PR do?

It initializes runtimeMetricsV2 with the "direct" client, which provides the DistributionSamples() method (in addition to all the usual statsd.Client methods).

Motivation

See DataDog/go-runtime-metrics-internal#8.

The Go runtime/metrics package maintains histograms, and we want to submit those to the Datadog API as distributions for best UX. We can roughly do so by submitting the midpoint of each bucket from the Go histogram, with a weight corresponding to the number of items in the bucket.

If we were to use the Distribution() method of the regular client, we would have to call it in a loop for the same bucket midpoint, which would be extremely wasteful for buckets containing large number of items. Instead, the DistributionSamples() method allow us to rig the rate in order to achieve the same result, while sending only one point per bucket.

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: anatoleb/runtime-metrics-use-direct-client
Commit report: ae4e6a8
Test service: dd-trace-go

✅ 0 Failed, 5094 Passed, 67 Skipped, 2m 57.64s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-12-04 17:36:02

Comparing candidate commit 9beb947 in PR branch anatoleb/runtime-metrics-use-direct-client with baseline commit 076462a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 59 metrics, 0 unstable metrics.

[darccio]

@DataDog/data-streams-monitoring @DataDog/apm-idm-go Can you review this, please?