Clearer documentation and comments for enabling Prometheus support

agent/config/runtime.go

@@ -425,7 +425,10 @@ type RuntimeConfig struct {
 	// hcl: telemetry { dogstatsd_tags = []string }
 	TelemetryDogstatsdTags []string
 
-	// PrometheusRetentionTime enables prometheus Support for metrics if greater than 0
+	// PrometheusRetentionTime is the retention time for prometheus metrics if greater than 0.
+	// A value of 0 disable Prometheus support. Regarding Prometheus, it is considered a good
+	// practice to put large values here (such as a few days), and at least the interval between
+	// prometheus requests.
 	//
 	// hcl: telemetry { prometheus_retention_time = "duration" }
 	TelemetryPrometheusRetentionTime time.Duration

website/source/docs/agent/options.html.md

@@ -1332,8 +1332,11 @@ Consul will not enable TLS for the HTTP API unless the `https` port has been ass
     * <a name="telemetry-prometheus_retention_time"></a><a href="telemetry-prometheus_retention_time">prometheus_retention_time</a>
       If the value is greater than `0s` (the default), this enables [Prometheus](https://prometheus.io/) export of metrics.
       The duration can be expressed using the duration semantics and will aggregates all counters for the duration specified
-      (it might have an impact on Consul's memory usage).
-      Fetching the metrics using prometheus can then be performed using the `/v1/agent/metrics?format=prometheus`.
+      (it might have an impact on Consul's memory usage). A good value for this parameter is at least 2 times the interval of scrape
+      of Prometheus, but you might also put a very high retention time such as a few days (for instance 744h to enable retention
+      to 31 days).
+      Fetching the metrics using prometheus can then be performed using the `/v1/agent/metrics?format=prometheus` URL or by sending
+      the Accept header with value `text/plain; version=0.0.4; charset=utf-8`  to the `/v1/agent/metrics` (as done by Prometheus).
       The format is compatible natively with prometheus. When running in this mode, it is recommended to also enable the option
       <a href="#telemetry-disable_hostname">`disable_hostname`</a> to avoid having prefixed metrics with hostname.