net/dns/resolver: add ID to verbose logs in forwarder

To make it easier to correlate the starting/ending log messages.

Updates #cleanup

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I2802d53ad98e19bc8914bc58f8c04d4443227b26

net/dns/resolver/forwarder.go

@@ -460,6 +460,10 @@ func (f *forwarder) sendDoH(ctx context.Context, urlBase string, c *http.Client,
 var (
 	verboseDNSForward = envknob.RegisterBool("TS_DEBUG_DNS_FORWARD_SEND")
 	skipTCPRetry      = envknob.RegisterBool("TS_DNS_FORWARD_SKIP_TCP_RETRY")
+
+	// For correlating log messages in the send() function; only used when
+	// verboseDNSForward() is true.
+	forwarderCount atomic.Uint64
 )
 
 // send sends packet to dst. It is best effort.
@@ -467,9 +471,10 @@ var (
 // send expects the reply to have the same txid as txidOut.
 func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDelay) (ret []byte, err error) {
 	if verboseDNSForward() {
-		f.logf("forwarder.send(%q) ...", rr.name.Addr)
+		id := forwarderCount.Add(1)
+		f.logf("forwarder.send(%q) [%d] ...", rr.name.Addr, id)
 		defer func() {
-			f.logf("forwarder.send(%q) = %v, %v", rr.name.Addr, len(ret), err)
+			f.logf("forwarder.send(%q) [%d] = %v, %v", rr.name.Addr, id, len(ret), err)
 		}()
 	}
 	if strings.HasPrefix(rr.name.Addr, "http://") {