tracer: Parse global labels per tracer (#1290)

This patch removes the `globalLabel` variable and one-time parsing of
the global labels, in favor of parsing the global labels individually
on tracer construction. This has the advantage that tracers that are
created after start-up will parse environment variables at that time
and not only when the `apm` package is imported.

Signed-off-by: Marc Lopez Rubio <[email protected]>

CHANGELOG.asciidoc

@@ -23,6 +23,8 @@ endif::[]
 
 https://github.com/elastic/apm-agent-go/compare/v2.1.0...main[View commits]
 
+- Global labels are now parsed when the tracer is constructed, instead of parsing only once on package initialization {pull}1290[#(1290)]
+
 [[release-notes-2.x]]
 === Go Agent version 2.x
 

config.go

@@ -35,7 +35,6 @@ import (
 	"go.elastic.co/apm/v2/internal/apmlog"
 	"go.elastic.co/apm/v2/internal/configutil"
 	"go.elastic.co/apm/v2/internal/wildcard"
-	"go.elastic.co/apm/v2/model"
 	"go.elastic.co/apm/v2/transport"
 )
 
@@ -125,21 +124,6 @@ var (
 		"*auth*",
 		"set-cookie",
 	}, ","))
-
-	globalLabels = func() model.StringMap {
-		var labels model.StringMap
-		for _, kv := range configutil.ParseListEnv(envGlobalLabels, ",", nil) {
-			i := strings.IndexRune(kv, '=')
-			if i > 0 {
-				k, v := strings.TrimSpace(kv[:i]), strings.TrimSpace(kv[i+1:])
-				labels = append(labels, model.StringMapItem{
-					Key:   cleanLabelKey(k),
-					Value: truncateString(v),
-				})
-			}
-		}
-		return labels
-	}()
 )
 
 // Regular expression matching comment characters to escape in the User-Agent header value.

tracer.go

@@ -24,6 +24,7 @@ import (
 	"io"
 	"log"
 	"math/rand"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -159,6 +160,7 @@ type TracerOptions struct {
 	heapProfileInterval   time.Duration
 	exitSpanMinDuration   time.Duration
 	compressionOptions    compressionOptions
+	globalLabels          model.StringMap
 }
 
 // initDefaults updates opts with default values.
@@ -325,6 +327,7 @@ func (opts *TracerOptions) initDefaults(continueOnError bool) error {
 		log.Printf("[apm]: %s", err)
 	}
 
+	opts.globalLabels = parseGlobalLabels()
 	opts.requestDuration = requestDuration
 	opts.metricsInterval = metricsInterval
 	opts.requestSize = requestSize
@@ -407,6 +410,7 @@ type Tracer struct {
 	breakdownMetrics  *breakdownMetrics
 	profileSender     profileSender
 	versionGetter     majorVersionGetter
+	globalLabels      model.StringMap
 
 	// stats is heap-allocated to ensure correct alignment for atomic access.
 	stats *TracerStats
@@ -469,6 +473,7 @@ func newTracer(opts TracerOptions) *Tracer {
 		instrumentationConfigInternal: &instrumentationConfig{
 			local: make(map[string]func(*instrumentationConfigValues)),
 		},
+		globalLabels: opts.globalLabels,
 	}
 	t.breakdownMetrics.enabled = opts.breakdownMetrics
 	// Initialise local transaction config.
@@ -1342,9 +1347,9 @@ func (t *Tracer) encodeRequestMetadata(json *fastjson.Writer) {
 		json.RawString(`,"cloud":`)
 		cloud.MarshalFastJSON(json)
 	}
-	if len(globalLabels) > 0 {
+	if len(t.globalLabels) > 0 {
 		json.RawString(`,"labels":`)
-		globalLabels.MarshalFastJSON(json)
+		t.globalLabels.MarshalFastJSON(json)
 	}
 	json.RawByte('}')
 }
@@ -1453,3 +1458,18 @@ type majorVersionGetter interface {
 	// cache is stale.
 	MajorServerVersion(ctx context.Context, refreshStale bool) uint32
 }
+
+func parseGlobalLabels() model.StringMap {
+	var labels model.StringMap
+	for _, kv := range configutil.ParseListEnv(envGlobalLabels, ",", nil) {
+		i := strings.IndexRune(kv, '=')
+		if i > 0 {
+			k, v := strings.TrimSpace(kv[:i]), strings.TrimSpace(kv[i+1:])
+			labels = append(labels, model.StringMapItem{
+				Key:   cleanLabelKey(k),
+				Value: truncateString(v),
+			})
+		}
+	}
+	return labels
+}