Add metrics reference page with examples

**What**
- Add reference documentation for the metrics exposed by the gateway
- Include examples for the usage of those metrics

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

Author: LucasRoesler

docs/reference/metrics.md

@@ -0,0 +1,48 @@
+# Monitoring Functions
+
+The Gateway component exposes several metrics to help you monitor the health and behavior of your functions
+
+| Metric                              | Type      | Description                    | Labels                  |
+| ----------------------------------- | --------- | ------------------------------ | ----------------------- |
+| `gateway_functions_seconds`         | histogram | Function invocation time taken | `function_name`         |
+| `gateway_function_invocation_total` | counter   | Function invocation count      | `function_name`, `code` |
+| `gateway_service_count`             | counter   | Number of function replicas    | `function_name`         |
+
+## Examples
+
+These basic metrics can be used to track the health of your functions as well a general usage patterns. See the Prometheus [documentation][prom-query-basics] and [examples][prom-query-examples] for more details about the available options and query functions. Below are several queries you might want to include in a basic [Grafana](https://grafana.com) dashboard for observing your OpenFaaS functions
+
+### Function invocation rate
+
+Return the per-second rate of invocation as measured over the previous 20 seconds:
+
+```
+rate ( gateway_function_invocation_total [20s])
+```
+
+### Function replica count / scaling
+
+Return the total function replicas:
+
+```
+gateway_service_count
+```
+
+### Total OK Function Invocation
+
+Return the total number of successful function invocations:
+
+```
+sum( gateway_function_invocation_total {  code=\"200\"}
+```
+
+### Function execution time
+
+Return the average function execution time, as measure over the previous 20 seconds:
+
+```
+(rate(gateway_functions_seconds_sum[20s]) / rate(gateway_functions_seconds_count[20s]))
+```
+
+[prom-query-basics]: https://prometheus.io/docs/prometheus/latest/querying/basics/
+[prom-query-examples]: https://prometheus.io/docs/prometheus/latest/querying/examples/