contrib/go-redis/redis.v8: optimize BeforeProcess and BeforeProcessPi…

…peline

This change includes the following three optimizations.

1. The previous implementation split the command with strings.Split, but only
   the 0th element is actually needed as a tag for span.
   Therefore, use strings.IndexByte to find the first space and slice the raw
   command string.

2. Replace the length derivation of args with the counting of the number of
   spaces.

3. The number of ddtrace.StartSpanOptions is often pre-determined, so make it
   in advance to reduce allocation.

contrib/go-redis/redis.v8/redis.go

@@ -100,16 +100,16 @@ func additionalTagOptions(client redis.UniversalClient) []ddtrace.StartSpanOptio
 
 func (ddh *datadogHook) BeforeProcess(ctx context.Context, cmd redis.Cmder) (context.Context, error) {
 	raw := cmd.String()
-	parts := strings.Split(raw, " ")
-	length := len(parts) - 1
+	length := strings.Count(raw, " ")
 	p := ddh.params
-	opts := []ddtrace.StartSpanOption{
+	opts := make([]ddtrace.StartSpanOption, 0, 5+len(ddh.additionalTags)+1) // 5 options below + for additionalTags + for analyticsRate
+	opts = append(opts,
 		tracer.SpanType(ext.SpanTypeRedis),
 		tracer.ServiceName(p.config.serviceName),
-		tracer.ResourceName(parts[0]),
+		tracer.ResourceName(raw[:strings.IndexByte(raw, ' ')]),
 		tracer.Tag("redis.raw_command", raw),
 		tracer.Tag("redis.args_length", strconv.Itoa(length)),
-	}
+	)
 	opts = append(opts, ddh.additionalTags...)
 	if !math.IsNaN(p.config.analyticsRate) {
 		opts = append(opts, tracer.Tag(ext.EventSampleRate, p.config.analyticsRate))
@@ -132,18 +132,18 @@ func (ddh *datadogHook) AfterProcess(ctx context.Context, cmd redis.Cmder) error
 
 func (ddh *datadogHook) BeforeProcessPipeline(ctx context.Context, cmds []redis.Cmder) (context.Context, error) {
 	raw := commandsToString(cmds)
-	parts := strings.Split(raw, " ")
-	length := len(parts) - 1
+	length := strings.Count(raw, " ")
 	p := ddh.params
-	opts := []ddtrace.StartSpanOption{
+	opts := make([]ddtrace.StartSpanOption, 0, 7+len(ddh.additionalTags)+1) // 7 options below + for additionalTags + for analyticsRate
+	opts = append(opts,
 		tracer.SpanType(ext.SpanTypeRedis),
 		tracer.ServiceName(p.config.serviceName),
-		tracer.ResourceName(parts[0]),
+		tracer.ResourceName(raw[:strings.IndexByte(raw, ' ')]),
 		tracer.Tag("redis.raw_command", raw),
 		tracer.Tag("redis.args_length", strconv.Itoa(length)),
 		tracer.Tag(ext.ResourceName, raw),
 		tracer.Tag("redis.pipeline_length", strconv.Itoa(len(cmds))),
-	}
+	)
 	opts = append(opts, ddh.additionalTags...)
 	if !math.IsNaN(p.config.analyticsRate) {
 		opts = append(opts, tracer.Tag(ext.EventSampleRate, p.config.analyticsRate))