upload session metrics

[butonic]

We added a gauge for the number of uploads currently in postprocessing as well as counters for different postprocessing outcomes:

	// UploadProcessing is the number of uploads in processing
	UploadProcessing = promauto.NewGauge(prometheus.GaugeOpts{
		Name: "reva_upload_processing",
		Help: "Number of uploads in processing",
	})
	// UploadSessionsInitiated is the number of upload sessions that have been initiated
	UploadSessionsInitiated = promauto.NewCounter(prometheus.CounterOpts{
		Name: "reva_upload_sessions_initiated",
		Help: "Number of uploads sessions that were initiated",
	})
	// UploadSessionsBytesReceived is the number of upload sessions that have received all bytes
	UploadSessionsBytesReceived = promauto.NewCounter(prometheus.CounterOpts{
		Name: "reva_upload_sessions_bytes_received",
		Help: "Number of uploads sessions that have received all bytes",
	})
	// UploadSessionsFinalized is the number of upload sessions that have received all bytes
	UploadSessionsFinalized = promauto.NewCounter(prometheus.CounterOpts{
		Name: "reva_upload_sessions_finalized",
		Help: "Number of uploads sessions that have successfully completed",
	})
	// UploadSessionsAborted is the number of upload sessions that have been aborted
	UploadSessionsAborted = promauto.NewCounter(prometheus.CounterOpts{
		Name: "reva_upload_sessions_aborted",
		Help: "Number of uploads sessions that have aborted by postprocessing",
	})
	// UploadSessionsDeleted is the number of upload sessions that have been deleted
	UploadSessionsDeleted = promauto.NewCounter(prometheus.CounterOpts{
		Name: "reva_upload_sessions_deleted",
		Help: "Number of uploads sessions that have been deleted by postprocessing",
	})
	// UploadSessionsRestarted is the number of upload sessions that have been restarted
	UploadSessionsRestarted = promauto.NewCounter(prometheus.CounterOpts{
		Name: "reva_upload_sessions_restarted",
		Help: "Number of uploads sessions that have been restarted by postprocessing",
	})
	// UploadSessionsScanned is the number of upload sessions that have been scanned by antivirus
	UploadSessionsScanned = promauto.NewCounter(prometheus.CounterOpts{
		Name: "reva_upload_sessions_scanned",
		Help: "Number of uploads sessions that have been scanned by antivirus",
	})
)