Add documentation for the new prometheus exporter feature (#1589)

See https://github.com/mastodon/mastodon/pull/33734

content/en/admin/config.md

@@ -645,14 +645,6 @@ The password used to authenticate with sentinels.
 
 If set to `true`, Mastodon will use Elasticsearch for its search functions.
 
-### Elasticsearch {#elasticsearch}
-
-{{< page-ref page="admin/elasticsearch" >}}
-
-#### `ES_ENABLED`
-
-If set to `true`, Mastodon will use Elasticsearch for its search functions.
-
 #### `ES_PRESET`
 
 It controls the Elasticsearch indices configuration (number of shards and replica).
@@ -692,26 +684,6 @@ Override Certificate Authority bundle file to use. Useful when using self-signed
 **Version history:**\
 4.3.0 - added
 
-### OpenTelemetry {#otel}
-
-Mastodon supports exporting tracing data using the OpenTelemetry protocol. The instrumentation uses the standard OTEL Ruby SDK, and should support the [standard OTEL environment configuration variables](https://opentelemetry.io/docs/languages/sdk-configuration/general/), with the exception of `OTEL_SERVICE_NAME` (see `OTEL_SERVICE_NAME_PREFIX` below). Mastodon currently only ships with the OLTP exporter.
-
-**Version history:**\
-4.3.0 - added support for the Ruby backend
-
-#### `OTEL_SERVICE_NAME_PREFIX`
-
-Prefix for the OTEL service names. The services names will be `$prefix/web` and `$prefix/sidekiq`. Defaults to `mastodon`.
-
-#### `OTEL_SERVICE_NAME_SEPARATOR`
-
-What character to use in service names when differentiating between different services. Defaults to `/` (i.e. `mastodon/web`).
-
-
-#### `OTEL_EXPORTER_OTLP_ENDPOINT`
-
-URL of the OLTP server to send the traces to. OpenTelemetry instrumentation is disabled if this variable is not set. No default (empty value).
-
 ### SMTP email delivery {#smtp}
 
 #### `SMTP_SERVER`
@@ -774,6 +746,73 @@ By default, a StartTLS connection will be attempted to the specified SMTP server
 
 Note that `TLSv1.3` and `TLSv1.2` are the only SSL/TLS protocols currently considered to be secure.
 
+### Prometheus Metrics {#prometheus}
+
+Mastodon optionally supports exposing some metrics using the Prometheus format.
+
+For the Ruby processes, it is using the [`prometheus_exporter` gem](https://github.com/discourse/prometheus_exporter). Please refer to their documentation for more details.
+
+By default, you will need to run a `prometheus_exporter` server (using `./bin/prometheus_exporter`) to collect the metrics and expose them to be scraped. See `MASTODON_PROMETHEUS_EXPORTER_LOCAL` if you want to change this behaviour.
+
+Note that metrics in the Prometheus format are always enabled for the streaming server, and can be accessed at `http://streaming-server-host:port/metrics`
+
+**Version history:**\
+4.4.0 - added support for the Ruby processes
+
+#### `MASTODON_PROMETHEUS_EXPORTER_ENABLED`
+
+If set to `true`, Mastodon's Ruby processes (web & Sidekiq) will enable the Prometheus instrumentation.
+
+#### `MASTODON_PROMETHEUS_EXPORTER_WEB_DETAILED_METRICS`
+
+If set to `true`, the instrumentation will collect and expose per-controller/action metrics for every web request. Note that this might cause some resource overhead.
+
+#### `MASTODON_PROMETHEUS_EXPORTER_SIDEKIQ_DETAILED_METRICS`
+
+If set to `true`, the instrumentation will collect and expose per job metrics for every Sidekiq job. Note that this might cause some resource overhead.
+
+#### `MASTODON_PROMETHEUS_EXPORTER_LOCAL`
+
+If set to `true`, an in-process server will be started to expose the metrics, rather than trying to send them to an external `prometheus_exporter` server. This can be useful when running Sidekiq in a containerized environment to avoid the overhead of the external exporter. Metrics will be exposed on `http://host:port/metrics`
+
+Important: this will not work for multi-process servers, like Puma, as every process will try to listen on the same port and will fail.
+
+#### `PROMETHEUS_EXPORTER_HOST`
+
+If the in-process server is not enabled, the metrics will be sent to this host (which should be running a `prometheus_exporter` server). Defaults to `localhost`.
+
+#### `PROMETHEUS_EXPORTER_PORT`
+
+If the in-process server is not enabled, the metrics will be sent to this host (which should be running a `prometheus_exporter` server). Defaults to `9394`.
+
+#### `MASTODON_PROMETHEUS_EXPORTER_HOST`
+
+If the in-process server is enabled, the in-process exporter will listen on this host. Defaults to `localhost`
+
+#### `MASTODON_PROMETHEUS_EXPORTER_PORT`
+
+If the in-process server is enabled, the in-process exporter will listen on this port. Defaults to `9394`
+
+### OpenTelemetry {#otel}
+
+Mastodon supports exporting tracing data using the OpenTelemetry protocol. The instrumentation uses the standard OTEL Ruby SDK, and should support the [standard OTEL environment configuration variables](https://opentelemetry.io/docs/languages/sdk-configuration/general/), with the exception of `OTEL_SERVICE_NAME` (see `OTEL_SERVICE_NAME_PREFIX` below). Mastodon currently only ships with the OLTP exporter.
+
+**Version history:**\
+4.3.0 - added support for the Ruby backend
+
+#### `OTEL_SERVICE_NAME_PREFIX`
+
+Prefix for the OTEL service names. The services names will be `$prefix/web` and `$prefix/sidekiq`. Defaults to `mastodon`.
+
+#### `OTEL_SERVICE_NAME_SEPARATOR`
+
+What character to use in service names when differentiating between different services. Defaults to `/` (i.e. `mastodon/web`).
+
+
+#### `OTEL_EXPORTER_OTLP_ENDPOINT`
+
+URL of the OLTP server to send the traces to. OpenTelemetry instrumentation is disabled if this variable is not set. No default (empty value).
+
 ## File storage {#files}
 
 ### CDN {#cdn}