vendor: bump prometheus/client_golang v1.3.0

full diff: https://github.com/prometheus/client_golang/compare/v1.1.0...v1.3.0

This requires LK4D/vndr v0.1.0 or newer for vendoring; also adds a new
dependency: github.com/cespare/xxhash

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

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

@@ -0,0 +1,15 @@
+// +build appengine
+
+// This file contains the safe implementations of otherwise unsafe-using code.
+
+package xxhash
+
+// Sum64String computes the 64-bit xxHash digest of s.
+func Sum64String(s string) uint64 {
+	return Sum64([]byte(s))
+}
+
+// WriteString adds more data to d. It always returns len(s), nil.
+func (d *Digest) WriteString(s string) (n int, err error) {
+	return d.Write([]byte(s))
+}