p2.7: prometheus /metrics

Default GET /metrics is Prometheus text; JSON stays at /metrics.json
and ?format=json. :8443 needs the metrics caller bearer; loopback
stays open. Restricted callers have no per-caller series and no
tavily:restricted gauge.

vendor/github.com/cespare/xxhash/v2/xxhash_asm.go

@@ -0,0 +1,15 @@
+//go:build (amd64 || arm64) && !appengine && gc && !purego
+// +build amd64 arm64
+// +build !appengine
+// +build gc
+// +build !purego
+
+package xxhash
+
+// Sum64 computes the 64-bit xxHash digest of b with a zero seed.
+//
+//go:noescape
+func Sum64(b []byte) uint64
+
+//go:noescape
+func writeBlocks(d *Digest, b []byte) int