[litianningdatadog]

What does this PR do?

• LogFlusher::retry(Vec<RequestBuilder>): resends handed-back requests without draining the aggregator.
• with_policy(FlushPolicy) with retry_client_errors, return_additional_endpoint_failures, parallel_additional_endpoints and failure_logging: FailureLogging { PerAttempt (default), OncePerRequest }. OncePerRequest writes one error line per failed request, including the cause (status or transport error).
• #[non_exhaustive] on the new enums so variants can be added later.

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.

bottlecap resends a failed request once on the next flush; doing that with flush() would also drain new logs whose results are then discarded, so logs would be lost. It also retries most 4xx (403 stays terminal), hands back failures from additional endpoints, sends to all endpoints at once, and writes one error line per failed request: the extension re-ingests its own output, so a warning per attempt would add log volume.

Additional Notes

FlushPolicy::default() is today's behavior, so nothing changes for existing users.

Describe how to test/QA your changes

109 tests pass (new: retry doesn't drain, each policy switch, the full bottlecap policy, one-error-per-request logging with the cause). All sc-0 tests unchanged.

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)
    • [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 ← this PR
            • [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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