perf: improve memory usage in apmhttp module (#1531)

module/apmhttp/client.go

@@ -139,11 +139,11 @@ func (r *roundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
 func SetHeaders(req *http.Request, traceContext apm.TraceContext, propagateLegacyHeader bool) {
 	headerValue := FormatTraceparentHeader(traceContext)
 	if propagateLegacyHeader {
-		req.Header.Set(ElasticTraceparentHeader, headerValue)
+		req.Header[ElasticTraceparentHeader] = []string{headerValue}
 	}
-	req.Header.Set(W3CTraceparentHeader, headerValue)
+	req.Header[W3CTraceparentHeader] = []string{headerValue}
 	if tracestate := traceContext.State.String(); tracestate != "" {
-		req.Header.Set(TracestateHeader, tracestate)
+		req.Header[TracestateHeader] = []string{tracestate}
 	}
 }
 

module/apmhttp/requestname.go

@@ -19,36 +19,21 @@ package apmhttp // import "go.elastic.co/apm/module/apmhttp/v2"
 
 import (
 	"net/http"
-	"strings"
 )
 
 // UnknownRouteRequestName returns the transaction name for the server request, req,
 // when the route could not be determined.
 func UnknownRouteRequestName(req *http.Request) string {
 	const suffix = " unknown route"
-	var b strings.Builder
-	b.Grow(len(req.Method) + len(suffix))
-	b.WriteString(req.Method)
-	b.WriteString(suffix)
-	return b.String()
+	return req.Method + suffix
 }
 
 // ServerRequestName returns the transaction name for the server request, req.
 func ServerRequestName(req *http.Request) string {
-	var b strings.Builder
-	b.Grow(len(req.Method) + len(req.URL.Path) + 1)
-	b.WriteString(req.Method)
-	b.WriteByte(' ')
-	b.WriteString(req.URL.Path)
-	return b.String()
+	return req.Method + " " + req.URL.Path
 }
 
 // ClientRequestName returns the span name for the client request, req.
 func ClientRequestName(req *http.Request) string {
-	var b strings.Builder
-	b.Grow(len(req.Method) + len(req.URL.Host) + 1)
-	b.WriteString(req.Method)
-	b.WriteByte(' ')
-	b.WriteString(req.URL.Host)
-	return b.String()
+	return req.Method + " " + req.URL.Host
 }

module/apmhttp/traceheaders.go

@@ -19,7 +19,7 @@ package apmhttp // import "go.elastic.co/apm/module/apmhttp/v2"
 
 import (
 	"encoding/hex"
-	"fmt"
+	"strconv"
 	"strings"
 
 	"github.com/pkg/errors"
@@ -52,8 +52,18 @@ const (
 // FormatTraceparentHeader formats the given trace context as a
 // traceparent header.
 func FormatTraceparentHeader(c apm.TraceContext) string {
-	const version = 0
-	return fmt.Sprintf("%02x-%032x-%016x-%02x", 0, c.Trace[:], c.Span[:], c.Options)
+	var traceId [32]byte
+	hex.Encode(traceId[:], c.Trace[:])
+
+	var spanId [16]byte
+	hex.Encode(spanId[:], c.Span[:])
+
+	opts := strconv.FormatUint(uint64(c.Options), 16)
+	if len(opts) == 1 {
+		opts = "0" + opts
+	}
+
+	return "00-" + string(traceId[:]) + "-" + string(spanId[:]) + "-" + opts
 }
 
 // ParseTraceparentHeader parses the given header, which is expected to be in

module/apmhttp/traceheaders_test.go

@@ -89,3 +89,17 @@ func TestParseTracestateHeader(t *testing.T) {
 	tracestate, _ = assertParse("vendorname1=opaqueValue1", "vendorname2=opaqueValue2")
 	assert.Equal(t, "vendorname1=opaqueValue1,vendorname2=opaqueValue2", tracestate.String())
 }
+
+func BenchmarkTraceHeaders(b *testing.B) {
+	ctx := apm.TraceContext{
+		Trace:   apm.TraceID{1},
+		Span:    apm.SpanID{2},
+		Options: apm.TraceOptions(3),
+	}
+
+	b.Run("new", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			apmhttp.FormatTraceparentHeader(ctx)
+		}
+	})
+}

tracecontext.go

@@ -349,5 +349,5 @@ func formatElasticTracestateValue(sampleRate float64) string {
 	// 0       -> "s:0"
 	// 1       -> "s:1"
 	// 0.55555 -> "s:0.5555" (any rounding should be applied prior)
-	return fmt.Sprintf("s:%.4g", sampleRate)
+	return "s:" + strconv.FormatFloat(sampleRate, 'g', 4, 64)
 }

transaction.go

@@ -50,7 +50,7 @@ func (t *Tracer) StartTransactionOptions(name, transactionType string, opts Tran
 				captureBodyMask: CaptureBodyTransactions,
 			},
 			spanTimings:       make(spanTimingsMap),
-			droppedSpansStats: make(droppedSpanTimingsMap, maxDroppedSpanStats),
+			droppedSpansStats: make(droppedSpanTimingsMap),
 		}
 		var seed int64
 		if err := binary.Read(cryptorand.Reader, binary.LittleEndian, &seed); err != nil {