metrics: Add the number of requests in the waiting queue (#11580)

We can use this metric to check if there are too many S3 clients in the
queue and could explain why some of those S3 clients are timing out.

```
minio_s3_requests_waiting_total{server="127.0.0.1:9000"} 9981
```

If max_requests is 10000 then there is a strong possibility that clients
are timing out because of the queue deadline.

cmd/admin-handlers.go

@@ -259,6 +259,7 @@ type ServerHTTPAPIStats struct {
 // ServerHTTPStats holds all type of http operations performed to/from the server
 // including their average execution time.
 type ServerHTTPStats struct {
+	S3RequestsInQueue int32              `json:"s3RequestsInQueue"`
 	CurrentS3Requests ServerHTTPAPIStats `json:"currentS3Requests"`
 	TotalS3Requests   ServerHTTPAPIStats `json:"totalS3Requests"`
 	TotalS3Errors     ServerHTTPAPIStats `json:"totalS3Errors"`