docs: [Marathon SD] Add Marathon service discovery doc

docs/changelog/CHANGELOG.md

@@ -22,6 +22,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent/): allow using HTTP/2 client for Kubernetes service discovery if `-promscrape.kubernetes.useHTTP2Client` cmd-line flag is set. This could help to reduce the amount of opened connections to the Kubernetes API server. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5971) for the details.
 * FEATURE: [vmalert](https://docs.victoriametrics.com/vmalert/): `-rule` cmd-line flag now supports multi-document YAML files. This could be useful when rules are retrieved via HTTP URL where multiple rule files were merged together in one response. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6753). Thanks to @Irene-123 for [the pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6995).
 * FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent/) and [Single-node VictoriaMetrics](https://docs.victoriametrics.com/): add support of [exponential histograms](https://opentelemetry.io/docs/specs/otel/metrics/data-model/#exponentialhistogram) ingested via [OpenTelemetry protocol for metrics](https://docs.victoriametrics.com/#sending-data-via-opentelemetry). Such histograms will be automatically converted to [VictoriaMetrics histogram format](https://valyala.medium.com/improving-histogram-usability-for-prometheus-and-grafana-bc7e5df0e350). See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6354).
+* FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent/): add service discovery support for [Marathon](https://mesosphere.github.io/marathon/). See [these docs](https://docs.victoriametrics.com/sd_configs/#marathon_sd_configs) and [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6642).
 
 ## [v1.104.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.104.0)
 

docs/sd_configs.md

@@ -28,6 +28,7 @@ supports the following Prometheus-compatible service discovery options for Prome
 * `http_sd_configs` is for discovering and scraping targets provided by external http-based service discovery. See [these docs](#http_sd_configs).
 * `kubernetes_sd_configs` is for discovering and scraping [Kubernetes](https://kubernetes.io/) targets. See [these docs](#kubernetes_sd_configs).
 * `kuma_sd_configs` is for discovering and scraping [Kuma](https://kuma.io) targets. See [these docs](#kuma_sd_configs).
+* `marathon_sd_configs` is for discovering and scraping [Marathon](https://mesosphere.github.io/marathon/) targets. See [these docs](#marathon_sd_configs).
 * `nomad_sd_configs` is for discovering and scraping targets registered in [HashiCorp Nomad](https://www.nomadproject.io/). See [these docs](#nomad_sd_configs).
 * `openstack_sd_configs` is for discovering and scraping OpenStack targets. See [these docs](#openstack_sd_configs).
 * `ovhcloud_sd_configs` is for discovering and scraping OVH Cloud VPS and dedicated server targets. See [these docs](#ovhcloud_sd_configs).
@@ -1266,6 +1267,43 @@ The following meta labels are available on discovered targets during [relabeling
 
 The list of discovered Kuma targets is refreshed at the interval, which can be configured via `-promscrape.kumaSDCheckInterval` command-line flag.
 
+## marathon_sd_configs
+
+_Available from [CHANGEME](https://docs.victoriametrics.com/changelog/#vCHANGEME) version._
+
+Marathon SD configuration allows retrieving scrape targets from [Marathon](https://mesosphere.github.io/marathon/) REST API.
+
+Configuration example:
+
+```yaml
+scrape_configs:
+- job_name: nomad
+  marathon_sd_configs:
+    # List of URLs to be used to contact Marathon servers.
+    # You need to provide at least one server URL, but should provide URLs for
+    # all masters you have running.
+    #
+    servers:
+      - "host1:port1"
+      - "host2:port2"
+      - "..."
+    
+    # Additional HTTP API client options can be specified here.
+    # See https://docs.victoriametrics.com/sd_configs/#http-api-client-options
+```
+
+The following meta labels are available on discovered targets during [relabeling](https://docs.victoriametrics.com/vmagent/#relabeling):
+
+* `__meta_marathon_app`: the name of the app (with slashes replaced by dashes)
+* `__meta_marathon_image`: the name of the Docker image used (if available)
+* `__meta_marathon_task`: the ID of the Mesos task
+* `__meta_marathon_app_label_<labelname>`: any Marathon labels attached to the app
+* `__meta_marathon_port_definition_label_<labelname>`: the port definition labels
+* `__meta_marathon_port_mapping_label_<labelname>`: the port mapping labels
+* `__meta_marathon_port_index`: the port index number (e.g. `1` for `PORT1`)
+
+The list of discovered Marathon targets is refreshed at the interval, which can be configured via `-promscrape.marathonSDCheckInterval` command-line flag.
+
 ## nomad_sd_configs
 
 Nomad SD configuration allows retrieving scrape targets from [HashiCorp Nomad Services](https://www.hashicorp.com/blog/nomad-service-discovery).

docs/vmagent.md

@@ -1989,6 +1989,8 @@ See the docs at https://docs.victoriametrics.com/vmagent/ .
      Interval for checking for changes in Kubernetes API server. This works only if kubernetes_sd_configs is configured in '-promscrape.config' file. See https://docs.victoriametrics.com/sd_configs/#kubernetes_sd_configs for details (default 30s)
   -promscrape.kumaSDCheckInterval duration
      Interval for checking for changes in kuma service discovery. This works only if kuma_sd_configs is configured in '-promscrape.config' file. See https://docs.victoriametrics.com/sd_configs/#kuma_sd_configs for details (default 30s)
+  -promscrape.marathonSDCheckInterval duration
+     Interval for checking for changes in Marathon service discovery. This works only if marathon_sd_configs is configured in '-promscrape.config' file. See https://docs.victoriametrics.com/sd_configs.html#marathon_sd_configs for details  (default 30s)
   -promscrape.maxDroppedTargets int
      The maximum number of droppedTargets to show at /api/v1/targets page. Increase this value if your setup drops more scrape targets during relabeling and you need investigating labels for all the dropped targets. Note that the increased number of tracked dropped targets may result in increased memory usage (default 10000)
   -promscrape.maxResponseHeadersSize size