Refactor docs

docs/metrics.md

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+# Metrics
+
+SFTPGo exposes [Prometheus](https://prometheus.io/) metrics at the `/metrics` HTTP endpoint.
+Several counters and gauges are available, for example:
+
+- Total uploads and downloads
+- Total upload and download size
+- Total upload and download errors
+- Total executed SSH commands
+- Total SSH command errors
+- Number of active connections
+- Data provider availability
+- Total successful and failed logins using password, public key or keyboard interactive authentication
+- Total HTTP requests served and totals for response code
+- Go's runtime details about GC, number of gouroutines and OS threads
+- Process information like CPU, memory, file descriptor usage and start time
+
+Please check the `/metrics` page for more details.