feat: add per-user write byte statistics

[davidby-influx]

Add a userwritebytes statistic, the write-path mirror of the shipped userquerybytes measurement, gated behind a new opt-in [http] user-write-bytes-enabled flag. When enabled, every v1, v2, and Prometheus remote write adds its request body bytes to a per-user counter, reported as one userwritebytes statistic per user, tagged with the user name, and visible through SHOW STATS and /debug/vars (admin-gated only when pprof-auth-enabled is set; otherwise /debug/vars is unauthenticated). Unauthenticated writes are attributed to "(anonymous)". Bytes are counted at the same sites that feed the aggregate writeReqBytes field of the httpd statistic, so the per-user values sum exactly to the global counter. Behavior is unchanged by default.

The counters inherit the existing writeReqBytes units: /write counts post-gzip decompressed bytes while the Prometheus endpoint counts compressed wire bytes, and tests pin both so changing that inconsistency becomes a conscious decision. New tests cover the sum invariant, anonymous attribution, concurrent writes under the race detector, early-error paths that must count nothing, and /debug/vars visibility under both pprof-auth modes (admin-gated and the unauthenticated default). The test harness gains a WriteAuthorizer mock, previously declared but never wired, making authenticated write paths testable for the first time. The per-user statistic emission in Handler.Statistics is extracted into a helper shared with userquerybytes, and both flags are now documented in the sample config.

This also fixes a pre-existing gap where gzip request bodies bypassed the MaxBodySize limit: truncateReader now wraps the decompressed stream, so an oversized gzip body is rejected with 413 rather than buffered in full.

[Copilot]

Pull request overview

This PR adds opt-in per-user write request byte accounting to the httpd service (mirroring existing per-user query response byte tracking), and fixes MaxBodySize enforcement for gzip-encoded write bodies by applying truncation after decompression.

Changes:

• Add per-user userwritebytes statistics (tagged by user, with unauthenticated attributed to (anonymous)), gated by user-write-bytes-enabled.
• Ensure per-user write counters are incremented at the same points as the global writeReqBytes counter, preserving the “sum of per-user equals global” invariant.
• Fix MaxBodySize for gzip requests by wrapping the decoded stream with truncateReader, preventing decompression bombs from bypassing the limit; add extensive tests covering behavior and /debug/vars visibility.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
services/httpd/service.go	Adds the userWriteReqBytes value field constant for per-user write stats.
services/httpd/handler.go	Emits per-user userwritebytes stats and accounts write bytes per user; applies MaxBodySize after gzip decode.
services/httpd/handler_test.go	Adds comprehensive tests for per-user write byte accounting, gzip sizing behavior, concurrency, and /debug/vars exposure.
services/httpd/config.go	Introduces the user-write-bytes-enabled config flag.
etc/config.sample.toml	Documents the new user-write-bytes-enabled flag alongside the existing query counterpart.

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[davidby-influx]

https://github.com/influxdata/feature-requests/issues/692

[devanbenz]

LGTM

[devanbenz]

LGTM

[davidby-influx]

rebased