doc: Add metrics documentation

doc/metrics.md

@@ -0,0 +1,28 @@
+# Metrics collecting {#metrics}
+
+<!--
+Copyright 2022, Collabora, Ltd. and the Monado contributors
+SPDX-License-Identifier: BSL-1.0
+-->
+
+## Requirements
+
+Monado comes with it's own metrics collection and is built by default. No action
+is needed by the developer to build the Monado sides of things. You will need to
+checkout the [metrics repo][] to get the tools to analyse the resulting output
+from Monado.
+
+## Running
+
+Launch the service with the env variable `XRT_METRICS_FILE` set.
+
+```bash
+XRT_METRICS_FILE=/path/to/file.protobuf monado-service
+```
+
+After Monado has finished running run the tool in the [metrics repo][], follow
+the instructions in the [README.md][] file inside of that repo, there are more
+instructions there.
+
+[metrics repo]: https://gitlab.freedesktop.org/monado/utilities/metrics
+[README.md]: https://gitlab.freedesktop.org/monado/utilities/metrics/-/blob/main/README.md