telemetry: Make http_user_agent a normalized field

This way we store a short 8-byte hash of the UA instead of the full
string; exactly the same way we store TLS ClientHello info.

caddyhttp/httpserver/mitm.go

@@ -65,9 +65,10 @@ func (h *tlsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	h.listener.helloInfosMu.RUnlock()
 
 	ua := r.Header.Get("User-Agent")
+	uaHash := telemetry.FastHash([]byte(ua))
 
 	// report this request's UA in connection with this ClientHello
-	go telemetry.AppendUnique("tls_client_hello_ua:"+caddytls.ClientHelloInfo(info).Key(), ua)
+	go telemetry.AppendUnique("tls_client_hello_ua:"+caddytls.ClientHelloInfo(info).Key(), uaHash)
 
 	var checked, mitm bool
 	if r.Header.Get("X-BlueCoat-Via") != "" || // Blue Coat (masks User-Agent header to generic values)

caddyhttp/httpserver/server.go

@@ -354,7 +354,9 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if len(ua) > 512 {
 		ua = ua[:512]
 	}
-	go telemetry.AppendUnique("http_user_agent", ua)
+	uaHash := telemetry.FastHash([]byte(ua)) // this is a normalized field
+	go telemetry.SetNested("http_user_agent", uaHash, ua)
+	go telemetry.AppendUnique("http_user_agent_count", uaHash)
 	go telemetry.Increment("http_request_count")
 
 	// copy the original, unchanged URL into the context

caddytls/handshake.go

@@ -516,7 +516,7 @@ func (info ClientHelloInfo) Key() string {
 	if !info.CompressionMethodsUnknown {
 		compressionMethods = fmt.Sprintf("%x", info.CompressionMethods)
 	}
-	return fastHash([]byte(fmt.Sprintf("%x-%x-%s-%s-%x-%x",
+	return telemetry.FastHash([]byte(fmt.Sprintf("%x-%x-%s-%s-%x-%x",
 		info.Version, info.CipherSuites, extensions,
 		compressionMethods, info.Curves, info.Points)))
 }

telemetry/collection.go

@@ -15,6 +15,8 @@
 package telemetry
 
 import (
+	"fmt"
+	"hash/fnv"
 	"log"
 	"strings"
 
@@ -276,6 +278,15 @@ func atomicAdd(key string, amount int) {
 	bufferMu.Unlock()
 }
 
+// FastHash hashes input using a 32-bit hashing algorithm
+// that is fast, and returns the hash as a hex-encoded string.
+// Do not use this for cryptographic purposes.
+func FastHash(input []byte) string {
+	h := fnv.New32a()
+	h.Write(input)
+	return fmt.Sprintf("%x", h.Sum32())
+}
+
 // isDisabled returns whether key is
 // a disabled metric key. ALL collection
 // functions should call this and not