Add metrics example filtering on a single function

Signed-off-by: Lucas Roesler <[email protected]>

Author: LucasRoesler

docs/reference/metrics.md

@@ -44,5 +44,13 @@ Return the average function execution time, as measure over the previous 20 seco
 (rate(gateway_functions_seconds_sum[20s]) / rate(gateway_functions_seconds_count[20s]))
 ```
 
+### Metrics for a single function
+
+Each of the metrics generated by the Gateway are labeled with and can be filtered by the function name,  For example The invocation rate for just a single function (e.g. if the function name is `echo`) is given by
+
+```
+rate ( gateway_function_invocation_total{function_name='echo'} [20s])
+```
+
 [prom-query-basics]: https://prometheus.io/docs/prometheus/latest/querying/basics/
 [prom-query-examples]: https://prometheus.io/docs/prometheus/latest/querying/examples/