Update docs

Author: kalleep

docs/sources/reference/components/loki/loki.write.md

@@ -201,6 +201,7 @@ The following fields are exported and can be referenced by other components:
 * `loki_write_dropped_entries_total` (counter): Number of log entries dropped because they failed to be sent to the ingester after all retries.
 * `loki_write_sent_bytes_total` (counter): Number of bytes sent.
 * `loki_write_sent_entries_total` (counter): Number of log entries sent to the ingester.
+* `loki_write_entry_propagation_latency` (histogram): Time in seconds from entry creation to successful send.
 * `loki_write_request_size_bytes` (histogram): Number of bytes for encoded requests.
 * `loki_write_request_duration_seconds` (histogram): Duration of sent requests.