docs(metrics): Update Grafana/Prometheus example

.examples/docker-compose-grafana-prometheus/README.md

@@ -1,23 +1,41 @@
 ## Usage
+Gatus exposes Prometheus metrics at `/metrics` if the `metrics` configuration option is set to `true`.
+
+To run this example, all you need to do is execute the following command:
 ```console
 docker-compose up
 ```
 Once you've done the above, you should be able to access the Grafana dashboard at `http://localhost:3000`.
 
-## Queries
-Gatus uses Prometheus counters.
+![Gatus Grafana dashboard](../../.github/assets/grafana-dashboard.png)
 
-Total results per minute:
+
+## Queries
+By default, this example has a Grafana dashboard with some panels, but for the sake of verbosity, you'll find
+a list of simple queries below. Those make use of the `key` parameter, which is a concatenation of the endpoint's
+group and name.
+
+### Success rate
+```
+sum(rate(gatus_results_total{success="true"}[30s])) by (key) / sum(rate(gatus_results_total[30s])) by (key)
+```
+
+### Response time
+```
+gatus_results_duration_seconds
+```
+
+### Total results per minute
 ```
 sum(rate(gatus_results_total[5m])*60) by (key)
 ```
 
-Total successful results per minute:
+### Total successful results per minute
 ```
 sum(rate(gatus_results_total{success="true"}[5m])*60) by (key)
 ```
 
-Total unsuccessful results per minute:
+### Total unsuccessful results per minute
+```
+sum(rate(gatus_results_total{success="false"}[5m])*60) by (key)
 ```
-sum(rate(gatus_results_total{success="true"}[5m])*60) by (key)
-```