Add prometheus metric requests_total with headers

Co-authored-by: Julien Salleyron <julien.salleyron@gmail.com>

docs/content/observability/metrics/prometheus.md

@@ -165,3 +165,66 @@ metrics:
 ```bash tab="CLI"
 --metrics.prometheus.manualrouting=true
 ```
+
+#### `headerLabels`
+
+_Optional_
+
+Defines the extra labels for the `requests_total` metrics, and for each of them, the request header containing the value for this label.
+Please note that if the header is not present in the request it will be added nonetheless with an empty value.
+In addition, the label should be a valid label name for Prometheus metrics, 
+otherwise, the Prometheus metrics provider will fail to serve any Traefik-related metric.
+
+```yaml tab="File (YAML)"
+metrics:
+  prometheus:
+    headerLabels:
+      label: headerKey
+```
+
+```toml tab="File (TOML)"
+[metrics]
+  [metrics.prometheus]
+    [metrics.prometheus.headerLabels]
+      label = "headerKey"
+```
+
+```bash tab="CLI"
+--metrics.prometheus.headerlabels.label=headerKey
+```
+
+##### Example
+
+Here is an example of the entryPoint `requests_total` metric with an additional "useragent" label.
+
+When configuring the label in Static Configuration:
+
+```yaml tab="File (YAML)"
+metrics:
+  prometheus:
+    headerLabels:
+      useragent: User-Agent
+```
+
+```toml tab="File (TOML)"
+[metrics]
+  [metrics.prometheus]
+    [metrics.prometheus.headerLabels]
+      useragent = "User-Agent"
+```
+
+```bash tab="CLI"
+--metrics.prometheus.headerlabels.useragent=User-Agent
+```
+
+And performing a request with a custom User-Agent:
+
+```bash
+curl -H "User-Agent: foobar" http://localhost
+```
+
+The following metric is produced :
+
+```bash
+traefik_entrypoint_requests_total{code="200",entrypoint="web",method="GET",protocol="http",useragent="foobar"} 1
+```

docs/content/reference/static-configuration/cli-ref.md

@@ -354,6 +354,9 @@ Buckets for latency metrics. (Default: ```0.100000, 0.300000, 1.200000, 5.000000
 `--metrics.prometheus.entrypoint`:  
 EntryPoint (Default: ```traefik```)
 
+`--metrics.prometheus.headerlabels.<name>`:  
+Defines the extra labels for the requests_total metrics, and for each of them, the request header containing the value for this label.
+
 `--metrics.prometheus.manualrouting`:  
 Manual routing (Default: ```false```)
 

docs/content/reference/static-configuration/env-ref.md

@@ -354,6 +354,9 @@ Buckets for latency metrics. (Default: ```0.100000, 0.300000, 1.200000, 5.000000
 `TRAEFIK_METRICS_PROMETHEUS_ENTRYPOINT`:  
 EntryPoint (Default: ```traefik```)
 
+`TRAEFIK_METRICS_PROMETHEUS_HEADERLABELS_<NAME>`:  
+Defines the extra labels for the requests_total metrics, and for each of them, the request header containing the value for this label.
+
 `TRAEFIK_METRICS_PROMETHEUS_MANUALROUTING`:  
 Manual routing (Default: ```false```)
 

docs/content/reference/static-configuration/file.toml

@@ -272,6 +272,9 @@
     addServicesLabels = true
     entryPoint = "foobar"
     manualRouting = true
+    [metrics.prometheus.headerLabels]
+      label1 = "foobar"
+      label2 = "foobar"
   [metrics.datadog]
     address = "foobar"
     pushInterval = "42s"

docs/content/reference/static-configuration/file.yaml

@@ -298,6 +298,9 @@ metrics:
     addServicesLabels: true
     entryPoint: foobar
     manualRouting: true
+    headerLabels:
+      label1: foobar
+      label2: foobar
   datadog:
     address: foobar
     pushInterval: 42s