fix(logging): port upstream slog-leveler fixes

[puertomontt]

Description

pkg/logging is derived from slog-leveler. Upstream PR #6 (0752d55) is the only change since we forked; this ports the parts that apply to us.

The bug: HTTPLevelHandler mutated component levels as it iterated the parsed request, so a request naming an unknown component was applied partially. Because Go randomizes map iteration order, the same request returned 200 or 400 nondeterministically — the same input, eight times:

attempt 0: status=400 level(probe-a)=INFO  body="logger not found for component: does-not-exist\n"
attempt 1: status=200 level(probe-a)=DEBUG body="component probe-a log level set to: debug\nlogger not found for component: does-not-exist\n"
attempt 6: status=400 level(probe-a)=INFO  body="logger not found for component: does-not-exist\n"
attempt 7: status=200 level(probe-a)=DEBUG body="component probe-a log level set to: debug\nlogger not found for component: does-not-exist\n"

On the 200 path the known component's level is changed despite the request failing, and http.Error fires after w.Write has already committed the status, so the response carries both a success line and an error line. Note that the bad-level case was already atomic (parsed in an earlier loop); only unknown components leaked. Fixed by validating every requested component up front, then mutating.

Also in this PR:

• Remove the exported GlobalLevel. It was documented as "the slog.LevelVar for the default logger" but was never stored in componentLeveler and never read outside a test that compared it against its own zero value. Upstream deleted it in the same PR. Verified zero references repo-wide.
• Reset the componentLeveler map between tests rather than only resetting levels, so components no longer leak across cases.
• Fix the HTTPLelevelHandler doc typo, and document that DeleteLeveler leaves existing loggers working but disconnected from SetLevel/GetLevel.

Not ported: upstream replaced init() with an explicit SetDefault() that main must call. That model assumes component loggers are not created as package-level vars, since those run before main and would find no registered default to inherit from — which is exactly our convention. init() stays.

Change Type

/kind fix

Changelog

NONE

Additional Notes

GlobalLevel is an exported symbol, so its removal is technically breaking for anything importing pkg/logging from outside the repo. It was inert, and upstream removed it, but happy to split it out if reviewers would rather keep the bug fix isolated.

Validation:

• go build -tags e2e ./... clean.
• go test ./pkg/logging/... -race -shuffle=on and ./pkg/kgateway/admin/... (the handler's only consumer) pass.
• Custom golangci-lint build over ./pkg/logging/...: 0 issues.
• The atomicity fix was confirmed against a temporary regression test that failed on the unpatched handler (level must be left untouched, expected: 0, actual: -4) and passed with the fix; the test was dropped from the final diff.

Follow-up, not in this PR: NewWithOptions computes AddSource: opts.AddSource || level.Level() <= slog.LevelDebug once at construction. Because nearly every logger in the repo is a package-level var initialized before SetupLogging applies the configured level, that clause resolves to false for all but controller-runtime and klog — so first-party component loggers never emit source even at debug. That clause is a kgateway-local addition; upstream has always used plain opts.AddSource.

[Copilot]

Pull request overview

This PR ports key fixes from upstream slog-leveler into kgateway’s pkg/logging, primarily to make HTTPLevelHandler’s component-level updates atomic (validate all requested components before mutating any levels) and to clean up related test and API surface issues.

Changes:

• Fix HTTPLevelHandler to validate all requested components up-front to avoid partial updates and nondeterministic success/failure responses.
• Remove the exported (but inert) GlobalLevel and adjust tests accordingly.
• Improve test isolation by fully resetting the componentLeveler map between test cases; update docs/typos.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
pkg/logging/logger.go	Clarifies DeleteLeveler behavior in docs (loggers remain usable but become disconnected from Set/Get).
pkg/logging/logger_test.go	Adds resetComponentLeveler() helper and uses it to prevent cross-test pollution.
pkg/logging/level.go	Removes GlobalLevel, fixes handler comment typo, and makes HTTPLevelHandler validate components before mutating levels.
pkg/logging/level_test.go	Updates expectations for GlobalLevel removal and uses resetComponentLeveler() for stronger test isolation.