Emit latency traces when sampling is set to headers

Latency reports will include the header(s) responsible for the trace
Updated ADR to have it reflect implementation

Signed-off-by: Matthias Hanel <mh@synadia.com>

server/parser.go

@@ -14,7 +14,11 @@
 package server
 
 import (
+	"bufio"
+	"bytes"
 	"fmt"
+	"net/http"
+	"net/textproto"
 )
 
 type parserState int
@@ -26,6 +30,7 @@ type parseState struct {
 	pa      pubArg
 	argBuf  []byte
 	msgBuf  []byte
+	header  http.Header // access via getHeader
 	scratch [MAX_CONTROL_LINE_SIZE]byte
 }
 
@@ -442,7 +447,7 @@ func (c *client) parse(buf []byte) error {
 				c.traceMsg(c.msgBuf)
 			}
 			c.processInboundMsg(c.msgBuf)
-			c.argBuf, c.msgBuf = nil, nil
+			c.argBuf, c.msgBuf, c.header = nil, nil, nil
 			c.drop, c.as, c.state = 0, i+1, OP_START
 			// Drop all pub args
 			c.pa.arg, c.pa.pacache, c.pa.origin, c.pa.account, c.pa.subject = nil, nil, nil, nil, nil
@@ -1175,3 +1180,17 @@ func (c *client) clonePubArg() error {
 		}
 	}
 }
+
+func (ps *parseState) getHeader() http.Header {
+	if ps.header == nil {
+		if hdr := ps.pa.hdr; hdr > 0 {
+			reader := bufio.NewReader(bytes.NewReader(ps.msgBuf[0:hdr]))
+			tp := textproto.NewReader(reader)
+			tp.ReadLine() // skip over first line, contains version
+			if mimeHeader, err := tp.ReadMIMEHeader(); err == nil {
+				ps.header = http.Header(mimeHeader)
+			}
+		}
+	}
+	return ps.header
+}