feat(x402-metrics): add asset_symbol label for per-token queries

[bussyjd]

Summary

Adds asset_symbol to the 6 verifier metric vecs so per-token revenue is queryable in PromQL directly, without the frontend having to join metrics with the ServiceOffer CR's spec.payment.asset.symbol at query time.

Stack note: this is technically branched off fix/prom-retention-window-alignment (PR A) because that PR introduces pruneSeriesNotIn, the lastPaymentSuccess gauge, and x402-prometheus-rules.yaml — all of which this PR extends. Listed merge target is main per the stacked-PR convention; once PR A lands, this should rebase cleanly. If reviewing before PR A merges, set the GitHub base to fix/prom-retention-window-alignment to see only the asset-symbol diff.

Cardinality math (no series multiplication)

Each ServiceOffer pins exactly one asset via spec.payment.asset.symbol. That means within the existing (offer_namespace, offer_name) group, asset_symbol is functionally constant — adding it does not multiply series count.

Per-vec series count before: 6 vecs * N offers * C chains-per-offer
Per-vec series count after: 6 vecs * N offers * C chains-per-offer * 1 asset-per-offer

The don't label what you can derive heuristic exists to prevent multiplicative blowups (chain x pod x pod_owner style). The single-asset-per-offer invariant means there's no multiplication to prevent — the same internal-consistency argument that already justifies chain (also CR-derived) on these vecs applies to asset_symbol.

Current test cluster (N=3 offers): demo-hello, silvermesh-paid, silvermesh-obol -> +3 series per vec, max +18 series total cluster-wide. Negligible.

Before / after PromQL

Before (frontend joins charged-requests metric with CR spec.payment.asset.symbol in JS):

sum by (offer_namespace, offer_name, chain) (
  increase(obol_x402_verifier_charged_requests_total[7d])
)
# then JS-side: foreach series, look up asset_symbol via /api/sell/list -> CR

After (direct):

sum by (asset_symbol) (x402:revenue:7d_by_offer_chain_asset_symbol)
# -> { asset_symbol="USDC" } 4231
# -> { asset_symbol="OBOL" } 88

Recording-rule migration plan

Non-breaking. Existing rules stay in place for one release:

• x402:revenue:24h_by_offer_chain -> kept (frontend currently reads this)
• x402:revenue:24h_by_offer_chain_asset_symbol -> new sibling
• x402:revenue:7d_by_offer_chain -> kept
• x402:revenue:7d_by_offer_chain_asset_symbol -> new sibling

Frontend can migrate to the _asset_symbol rules in a follow-up PR; the old rules continue to produce the same series (adding a label to the raw counter is non-breaking for queries that aggregate it away).

x402:settlement_rate:1h_by_offer_chain is intentionally not duplicated — it's a ratio; the asset wouldn't change the answer meaningfully.

Changes

• internal/x402/metrics.go -> 6 vecs gain asset_symbol; pruneSeriesNotIn key now 4-tuple
• internal/x402/verifier.go -> load() live-set uses 4-tuple; prometheusLabels() emits rule.AssetSymbol with "unknown" defensive fallback
• internal/embed/infrastructure/base/templates/x402-prometheus-rules.yaml -> two new _asset_symbol-suffixed recording rules
• internal/x402/verifier_test.go -> existing label-map assertions updated; 3 new tests added

Test plan

• go build ./... clean
• go vet ./internal/x402/... ./internal/embed/... clean
• go test ./internal/x402/... green (all 6 vecs + new tests pass)
• go test ./internal/embed/... green (CRD/manifest tests no regression)
• Existing TestVerifier_Reload_PrunesDeletedOfferSeries from PR A still passes — asset-symbol-aware key doesn't break basic GC
• TestVerifier_PrometheusLabels_IncludesAssetSymbol -> asserts USDC is emitted
• TestVerifier_PrometheusLabels_DefaultsToUnknownIfEmpty -> asserts "unknown" fallback when AssetSymbol is empty
• TestVerifier_PruneSeriesNotIn_DistinguishesAssetSymbol -> dropping USDC route while keeping OBOL route on the same offer leaves OBOL series alive (catches asset-repin leaks)
• Live-cluster verification: scrape obol_x402_verifier_charged_requests_total{asset_symbol="OBOL"} after a paid OBOL request to silvermesh-obol returns 1
• Recording rule rendered through helmfile: kubectl get prometheusrule -n x402 x402-verifier -o yaml shows both _by_offer_chain and _by_offer_chain_asset_symbol rules

[bussyjd]

Superseded by bundle PR #536 — closing in favor of the consolidated merge target. Original branch and history preserved.