Add Docs For Prometheus Metrics

website/source/docs/configuration/telemetry.html.md

@@ -135,3 +135,31 @@ These `telemetry` parameters apply to
 - `dogstatsd_tags` `(string array: [])` - This provides a list of global tags
   that will be added to all telemetry packets sent to DogStatsD. It is a list
   of strings, where each string looks like "my_tag_name:my_tag_value".
+
+### `prometheus`
+
+These `telemetry` parameters apply to
+[prometheus](https://prometheus.io).
+
+* `prometheus_retention_time` `(string: "24h")` - Specifies the amount of time that
+  prometheus metrics are retained in memory. It is recommended to also enable the option
+  `disable_hostname` to avoid having prefixed metrics with hostname. Vault does not use the
+  default Prometheus path, so Prometheus must be configured as follows. Note that using
+  ?format=prometheus in the path won't work as ? will be escaped, so it must be specified
+  as a parameter. Addtionally a Vault token is required to access /v1/sys/metrics. The prometheus
+  `bearer_token` or `bearer_token_file` options must be added to the scrape job.
+
+```
+metrics_path: "/v1/sys/metrics"
+params:
+  format: ['prometheus']
+bearer_token: your_vault_token_here
+
+```
+
+```hcl
+telemetry {
+  prometheus_retention_time = "30s",
+  disable_hostname = true
+}
+```