Add common middleware to S3 API handlers (#19171)

The middleware sets up tracing, throttling, gzipped responses and
collecting API stats.

Additionally, this change updates the names of handler functions in
metric labels to be the same as the name derived from Go lang reflection
on the handler name.

The metric api labels are now stored in memory the same as the handler
name - they will be camelcased, e.g. `GetObject` instead of `getobject`.

For compatibility, we lowercase the metric api label values when emitting the metrics.

cmd/api-utils.go

@@ -18,6 +18,10 @@
 package cmd
 
 import (
+	"fmt"
+	"net/http"
+	"reflect"
+	"runtime"
 	"strings"
 )
 
@@ -100,3 +104,16 @@ func s3EncodeName(name, encodingType string) string {
 	}
 	return name
 }
+
+// getHandlerName returns the name of the handler function. It takes the type
+// name as a string to clean up the name retrieved via reflection. This function
+// only works correctly when the type is present in the cmd package.
+func getHandlerName(f http.HandlerFunc, cmdType string) string {
+	name := runtime.FuncForPC(reflect.ValueOf(f).Pointer()).Name()
+
+	packageName := fmt.Sprintf("github.com/minio/minio/cmd.%s.", cmdType)
+	name = strings.TrimPrefix(name, packageName)
+	name = strings.TrimSuffix(name, "Handler-fm")
+	name = strings.TrimSuffix(name, "-fm")
+	return name
+}