[litianningdatadog]

What does this PR do?

Adds tests only (no production code) that pin what the crate does today with default settings:

• retry rules: 400/401/404/413 dropped after one try; 408/425/429 and connection errors tried three times and handed back; a redriven request that fails again is handed back again
• the default Datadog intake URL, headers and compression (on and off), and the 5 s request timeout
• additional endpoints: called after the primary by default, receive identical compressed bytes, keep their own API key, and are still sent when the primary fails (only the primary is handed back)
• how many warning lines are logged for each failure type, and that no error lines are written
• every from_env() default, and the 1 MiB entry limit (exactly 1 MiB kept, one byte more rejected)

Adds tracing-subscriber as a dev-dependency to count log lines.

Motivation

Part of SVLS-8713: bottlecap (the Datadog Lambda Extension) is replacing its own logs aggregator and flusher with this crate. Two rules for the whole stack:

• Nothing changes for existing users of this crate (for example serverless-compat): every new behavior is opt-in, defaults and public API stay the same (additions only), no new environment variables, no new dependencies.
• Nothing changes for Lambda users: the options let bottlecap reproduce its current behavior exactly.

The next PRs in this stack add options for bottlecap. These tests make sure none of them changes the defaults that other users rely on: if one does, a test fails.

Additional Notes

One commit (squashed from six per-area commits); the list above follows the same areas.

Describe how to test/QA your changes

cargo test -p datadog-logs-agent: 80 tests pass. clippy (-D warnings) and fmt clean.

Stack

Each PR is nested under the PR it builds on (and targets). Review and merge from the top down.

serverless-components (datadog-logs-agent options; merge first)

• main
  • 👉 [SVLS-8713] test(logs-agent): pin current default behavior with characterization tests #189 characterization tests (pin today's defaults) ← this PR
    • [SVLS-8713] build(logs-agent): let users choose the FIPS TLS backend #190 default / fips features
      • [SVLS-8713] feat(logs-agent): accept pre-serialized logs and an oversize policy #191 pre-serialized logs + oversize policy
        • [SVLS-8713] feat(logs-agent): optional API key resolver and Datadog intake URL override #192 API key resolver + intake URL override
          • [SVLS-8713] feat(logs-agent): add a retry-only method and an optional flush policy #193 retry() + FlushPolicy
            • [SVLS-8713] perf(logs-agent): compress each log batch only once #194 compress each batch once
              • [SVLS-8713] perf(logs-agent): compress log batches before waiting for the API key #195 compress before waiting for the API key

datadog-lambda-extension (bottlecap)

• main
  • [SVLS-8713] test(logs): contract tests, fake logs intake and benchmark for the logs pipeline datadog-lambda-extension#1448 contract tests, fake intake, benchmark
    • [SVLS-8713] fix(logs): send additional endpoints the Datadog intake format when OPW is on datadog-lambda-extension#1449 fix: OPW applies only to the primary endpoint
      • [SVLS-8713] fix(logs): drop a log too large for any payload instead of blocking the queue datadog-lambda-extension#1450 fix: drop a log too large for any payload
        • [SVLS-8713] test(logs): pin that a reloaded API key is used on the next flush datadog-lambda-extension#1451 pin: reloaded API key is used
          • bc-1: use the shared aggregator (opens after the serverless-components PRs merge)
            • bc-2: use the shared flusher (opens after the serverless-components PRs merge)

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