cache metrics: route revalidation, origin duration, Cache-Control - #4386

Summary of changes

Completes metrics with route revalidation outcomes, origin fetch duration, and Cache-Control directive tracking.

How to Verify

Shared setup

mkdir skipper-review && cd skipper-review
git clone https://github.com/zalando/skipper.git skipper && cd skipper
for PR in 4369 4370 4371 4372 4384 4385 4386 4387; do
  git fetch origin pull/$PR/head:pr-$PR
  git worktree add ../skipper-pr-$PR pr-$PR
done
cd ..
git clone https://github.com/larry-dalmeida/skipper-cache-e2e-tests.git
cd skipper-cache-e2e-tests
make verify SKIPPER_CHECKOUT=../skipper-pr-4387

make verify starts or reuses Minikube, the mock origin and the stack; runs the smoke and coalescing checks; and validates supported dashboard queries. It prints a report path and exits non-zero on failure. For outage evidence, run make chaos SKIPPER_CHECKOUT=../skipper-pr-4387 NAME=origin-stale; it includes its own timed load. Requirements: Python 3.9+, Make, Bash, Git, curl, kubectl, Minikube, Docker and Go compatible with Skipper's go.mod. Free ports: 8080, 8081, 6379, 4318, 9091, 9999, 9911 (8089 for endless load). Dynamic URLs: .runtime/urls.txt; stop with make stop.

Validated on #4387 (d767f636): smoke: 6,398 requests, 0 failures; origin-stale: 155 failed revalidations across 77 routes. Smoke metric checks: 42 PASS, 2 fault-only NOT EXERCISED, 0 FAIL.

Focused tests

go -C ../skipper-pr-4386 test -race -count=1 ./filters/cache -run '^(TestCacheOriginStatusClass|TestCacheRecordCacheControlBoundedLabels|TestCacheRecordCacheControlOncePerResponse|TestCacheFilterOriginMetrics|TestCacheRouteRevalDropped|TestCacheResponseOriginCacheControl|TestCacheRouteRevalFailurePaths|TestCacheRouteRevalTerminalOutcomes)$'
go -C ../skipper-pr-4386 test -race -count=1 ./metrics -run '^(TestPrometheusCacheRouteReval|TestPrometheusCacheOriginDurationRouteStatus|TestPrometheusCacheOriginCacheControl)$'
go -C ../skipper-pr-4386 test -race -count=1 ./metrics/metricstest -run '^TestCacheOriginMockMetrics$'

Expected: 12 focused tests pass; route-revalidation, origin-duration and bounded Cache-Control metrics emit samples. During origin-stale, origin 5xx increments failed (including cached errorTTL responses); legacy cache.reval_error now counts 5xx too and counts body-read failures once. Smoke alone does not exercise failures; see the shared outage report.

Checklist

• Added regression tests and documentation.
• Confirm CI passes on the current head.
• Confirm passess on manual run of e2e tests for all known scenarios including past bugs (results: larry-dalmeida/skipper-cache-e2e-tests/results/README.md)

Rollback Plan

Revert this PR to restore previous metrics behavior.

References & Additional Context

Depends on #4384 and #4385. Completes the metrics stack with route revalidation, origin latency, and Cache-Control directive metrics.

E2E Evidence

Latest-chain verification: origin-stale report records 155 failed revalidations across 77 routes; smoke report has 0 failures. The stack run uses #4387, which includes this PR.