Add metrics documentation

Co-authored-by: Tom Moulard <tom.moulard@traefik.io>

docs/content/observability/metrics/overview.md

@@ -1,6 +1,4 @@
 # Metrics
-Metrics system
-{: .subtitle }
 
 Traefik supports 4 metrics backends:
 
@@ -24,3 +22,306 @@ metrics: {}
 ```bash tab="CLI"
 --metrics=true
 ```
+
+## Server Metrics
+
+| Metric                                                                  | DataDog | InfluxDB | Prometheus | StatsD |
+|-------------------------------------------------------------------------|---------|----------|------------|--------|
+| [Configuration reloads](#configuration-reloads)                         | ✓       | ✓        | ✓          | ✓      |
+| [Configuration reload failures](#configuration-reload-failures)         | ✓       | ✓        | ✓          | ✓      |
+| [Last Configuration Reload Success](#last-configuration-reload-success) | ✓       | ✓        | ✓          | ✓      |
+| [Last Configuration Reload Failure](#last-configuration-reload-failure) | ✓       | ✓        | ✓          | ✓      |
+
+### Configuration Reloads
+The total count of configuration reloads.
+
+```dd tab="Datadog"
+config.reload.total
+```
+
+```influxdb tab="InfluDB"
+traefik.config.reload.total
+```
+
+```prom tab="Prometheus"
+traefik_config_reloads_total
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.config.reload.total
+```
+
+### Configuration Reload Failures
+The total count of configuration reload failures.
+
+```dd tab="Datadog"
+config.reload.total (with tag "failure" to true)
+```
+
+```influxdb tab="InfluDB"
+traefik.config.reload.total.failure
+```
+
+```prom tab="Prometheus"
+traefik_config_reloads_failure_total
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.config.reload.total.failure
+```
+
+### Last Configuration Reload Success
+The timestamp of the last configuration reload success.
+
+```dd tab="Datadog"
+config.reload.lastSuccessTimestamp
+```
+
+```influxdb tab="InfluDB"
+traefik.config.reload.lastSuccessTimestamp
+```
+
+```prom tab="Prometheus"
+traefik_config_last_reload_success
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.config.reload.lastSuccessTimestamp
+```
+
+### Last Configuration Reload Failure
+The timestamp of the last configuration reload failure.
+
+```dd tab="Datadog"
+config.reload.lastFailureTimestamp
+```
+
+```influxdb tab="InfluDB"
+traefik.config.reload.lastFailureTimestamp
+```
+
+```prom tab="Prometheus"
+traefik_config_last_reload_failure
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.config.reload.lastFailureTimestamp
+```
+
+## EntryPoint Metrics
+
+| Metric                                                    | DataDog | InfluxDB | Prometheus | StatsD |
+|-----------------------------------------------------------|---------|----------|------------|--------|
+| [HTTP Requests Count](#http-requests-count)               | ✓       | ✓        | ✓          | ✓      |
+| [HTTPS Requests Count](#https-requests-count)             |         |          | ✓          |        |
+| [Request Duration Histogram](#request-duration-histogram) | ✓       | ✓        | ✓          | ✓      |
+| [Open Connections Count](#open-connections-count)         | ✓       | ✓        | ✓          | ✓      |
+
+### HTTP Requests Count
+The total count of HTTP requests processed on an entrypoint.
+
+Available labels: `code`, `method`, `protocol`, `entrypoint`.
+
+```dd tab="Datadog"
+entrypoint.request.total
+```
+
+```influxdb tab="InfluDB"
+traefik.entrypoint.requests.total
+```
+
+```prom tab="Prometheus"
+traefik_entrypoint_requests_total
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.entrypoint.request.total
+```
+
+### HTTPS Requests Count
+The total count of HTTPS requests processed on an entrypoint.
+
+Available labels: `tls_version`, `tls_cipher`, `entrypoint`.
+
+```prom tab="Prometheus"
+traefik_entrypoint_requests_tls_total
+```
+
+### Request Duration Histogram
+Request process time duration histogram on an entrypoint.
+
+Available labels: `code`, `method`, `protocol`, `entrypoint`.
+
+```dd tab="Datadog"
+entrypoint.request.duration
+```
+
+```influxdb tab="InfluDB"
+traefik.entrypoint.request.duration
+```
+
+```prom tab="Prometheus"
+traefik_entrypoint_request_duration_seconds
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.entrypoint.request.duration
+```
+
+### Open Connections Count
+The current count of open connections on an entrypoint.
+
+Available labels: `method`, `protocol`, `entrypoint`.
+
+```dd tab="Datadog"
+entrypoint.connections.open
+```
+
+```influxdb tab="InfluDB"
+traefik.entrypoint.connections.open
+```
+
+```prom tab="Prometheus"
+traefik_entrypoint_open_connections
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.entrypoint.connections.open
+```
+
+## Service Metrics
+
+| Metric                                                      | DataDog | InfluxDB | Prometheus | StatsD |
+|-------------------------------------------------------------|---------|----------|------------|--------|
+| [HTTP Requests Count](#http-requests-count_1)               | ✓       | ✓        | ✓          | ✓      |
+| [HTTPS Requests Count](#https-requests-count_1)             |         |          | ✓          |        |
+| [Request Duration Histogram](#request-duration-histogram_1) | ✓       | ✓        | ✓          | ✓      |
+| [Open Connections Count](#open-connections-count_1)         | ✓       | ✓        | ✓          | ✓      |
+| [Requests Retries Count](#requests-retries-count)           | ✓       | ✓        | ✓          | ✓      |
+| [Service Server UP](#service-server-up)                     | ✓       | ✓        | ✓          | ✓      |
+
+### HTTP Requests Count
+The total count of HTTP requests processed on a service.
+
+Available labels: `code`, `method`, `protocol`, `service`.
+
+```dd tab="Datadog"
+service.request.total
+```
+
+```influxdb tab="InfluDB"
+traefik.service.requests.total
+```
+
+```prom tab="Prometheus"
+traefik_service_requests_total
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.service.request.total
+```
+
+### HTTPS Requests Count
+The total count of HTTPS requests processed on a service.
+
+Available labels: `tls_version`, `tls_cipher`, `service`.
+
+```prom tab="Prometheus"
+traefik_service_requests_tls_total
+```
+
+### Request Duration Histogram
+Request process time duration histogram on a service.
+
+Available labels: `code`, `method`, `protocol`, `service`.
+
+```dd tab="Datadog"
+service.request.duration
+```
+
+```influxdb tab="InfluDB"
+traefik.service.request.duration
+```
+
+```prom tab="Prometheus"
+traefik_service_request_duration_seconds
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.service.request.duration
+```
+
+### Open Connections Count
+The current count of open connections on a service.
+
+Available labels: `method`, `protocol`, `service`.
+
+```dd tab="Datadog"
+service.connections.open
+```
+
+```influxdb tab="InfluDB"
+traefik.service.connections.open
+```
+
+```prom tab="Prometheus"
+traefik_service_open_connections
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.service.connections.open
+```
+
+### Requests Retries Count
+The count of requests retries on a service.
+
+Available labels: `service`.
+
+```dd tab="Datadog"
+service.retries.total
+```
+
+```influxdb tab="InfluDB"
+traefik.service.retries.total
+```
+
+```prom tab="Prometheus"
+traefik_service_retries_total
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.service.retries.total
+```
+
+### Service Server UP
+Current service's server status, described by a gauge with a value of 0 for a down server or a value of 1 for an up server.
+
+Available labels: `service`, `url`.
+
+```dd tab="Datadog"
+service.server.up
+```
+
+```influxdb tab="InfluDB"
+traefik.service.server.up
+```
+
+```prom tab="Prometheus"
+traefik_service_server_up
+```
+
+```statsd tab="StatsD"
+# Default prefix: "traefik"
+{prefix}.service.server.up
+```