TUN-8731: Implement diag/system endpoint

## Summary
This PR will add a new endpoint, "diag/system" to the metrics server that collects system information from different operating systems.

Closes TUN-8731

diagnostic/error.go

@@ -0,0 +1,16 @@
+package diagnostic
+
+import (
+	"errors"
+)
+
+var (
+	// Error used when parsing the fields of the output of collector.
+	ErrInsufficientLines = errors.New("insufficient lines")
+	// Error used when parsing the lines of the output of collector.
+	ErrInsuficientFields = errors.New("insufficient fields")
+	// Error used when given key is not found while parsing KV.
+	ErrKeyNotFound = errors.New("key not found")
+	// Error used when tehre is no disk volume information available
+	ErrNoVolumeFound = errors.New("No disk volume information found")
+)