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/http-stats.go

@@ -143,6 +143,7 @@ type HTTPStats struct {
 	rejectedRequestsHeader  uint64
 	rejectedRequestsInvalid uint64
 	s3RequestsInQueue       int32
+	s3RequestsIncoming      uint64
 	currentS3Requests       HTTPAPIStats
 	totalS3Requests         HTTPAPIStats
 	totalS3Errors           HTTPAPIStats
@@ -153,9 +154,15 @@ func (st *HTTPStats) addRequestsInQueue(i int32) {
 	atomic.AddInt32(&st.s3RequestsInQueue, i)
 }
 
+func (st *HTTPStats) incS3RequestsIncoming() {
+	// Golang automatically resets to zero if this overflows
+	atomic.AddUint64(&st.s3RequestsIncoming, 1)
+}
+
 // Converts http stats into struct to be sent back to the client.
 func (st *HTTPStats) toServerHTTPStats() ServerHTTPStats {
 	serverStats := ServerHTTPStats{}
+	serverStats.S3RequestsIncoming = atomic.SwapUint64(&st.s3RequestsIncoming, 0)
 	serverStats.S3RequestsInQueue = atomic.LoadInt32(&st.s3RequestsInQueue)
 	serverStats.TotalS3RejectedAuth = atomic.LoadUint64(&st.rejectedRequestsAuth)
 	serverStats.TotalS3RejectedTime = atomic.LoadUint64(&st.rejectedRequestsTime)