prometheus: Add incoming requests metrics since last scrape (#14261)

Some users running MinIO claim that their system became slow. One 
way to investigate is to look at this Prometheus history of the number of
the requests reaching the server. The existing current S3 requests metric
is not enough because it can increase of the system really becomes slow, 
due to disk issues for example.

cmd/admin-handlers.go

@@ -284,6 +284,7 @@ type ServerHTTPAPIStats struct {
 // including their average execution time.
 type ServerHTTPStats struct {
 	S3RequestsInQueue      int32              `json:"s3RequestsInQueue"`
+	S3RequestsIncoming     uint64             `json:"s3RequestsIncoming"`
 	CurrentS3Requests      ServerHTTPAPIStats `json:"currentS3Requests"`
 	TotalS3Requests        ServerHTTPAPIStats `json:"totalS3Requests"`
 	TotalS3Errors          ServerHTTPAPIStats `json:"totalS3Errors"`