[larry-dalmeida]

Summary of changes

Adds LRU entry count and total capacity gauges for per-route cache monitoring.

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-4384 test -race -count=1 ./filters/cache -run '^TestShardedByteLRUUsage$'
go -C ../skipper-pr-4384 test -race -count=1 ./metrics -run '^TestPrometheusCacheLRUGauges$'

Expected: Both focused tests pass. Smoke shows entries grow and level off as route keys warm (about 990 entries/route); bytes remain far below the 2 GiB default (about 1 MB).

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

Base of metrics stack; followed by #4385. Adds skipper_cache_lru_entries (labelled by route) and skipper_cache_lru_capacity_bytes (the configured L1 limit) so L1 fill can be compared with capacity.

E2E Testing Results

The screenshots below come from a 2-minute smoke run (20261026-145513-smoke) against bda674b0, the top of this stack, which includes this PR. They show skipper_cache_lru_entries growing as keys are cached, and skipper_cache_lru_bytes plotted against the constant skipper_cache_lru_capacity_bytes (2 GiB). Full results: larry-dalmeida/skipper-cache-e2e-tests/results

Test Scenarios:

Scenario	Run	Skipper build	Requests	Failures	Status
Smoke (screenshots below)	20261026-145513-smoke	bda674b0 (top of this stack)	6,306	0%	✓ Pass
Smoke	20261025-201019-smoke	8a80d62e + uncommitted changes	6,270	0%	✓ Pass
Origin Stale	20261025-201220-origin-stale	8a80d62e + uncommitted changes	44,288	27% (expected during chaos)	✓ Pass
Skipper Kill	20261025-202222-skipper-kill	8a80d62e + uncommitted changes	38,969	1% (during restart)	✓ Pass

Validation Panels:

• Cache entries — skipper_cache_lru_entries climbs from 2 to about 860 entries during the 2-minute run as new keys are cached. It is still rising at the end, with no plateau in this run.
  

• Cache bytes vs capacity — skipper_cache_lru_capacity_bytes is a flat line at 2 GiB. skipper_cache_lru_bytes reaches only about 540 KiB, so at this scale it sits on the 0 B axis. L1 stays far below capacity.
  

• Cache hit ratio (context) — overall hit ratio for the run is 74.2%. Context only; this panel does not use a metric added by this PR.
  

[larry-dalmeida]

Superseded by #4395, stacked above #4394. Changes consolidated with published metric, dashboard and fault-test evidence.