Document URL parameter configuration for Prometheus

content/docs/operating/configuration.md

@@ -131,6 +131,10 @@ job_name: <name>
 # The URL scheme with which to fetch metrics from targets.
 [ scheme: <scheme> | default = http ]
 
+# Optional HTTP URL parameters.
+params:
+  [ <string>: [<string>, ...] ]
+
 # Optional authentication information. Note that `basic_auth`, `bearer_token`
 # `bearer_token_file` options are mutually exclusive.
 
@@ -518,7 +522,8 @@ label is set to the `job_name` value of the respective scrape configuration.
 The `__address__` label is set to the `<host>:<port>` address of the target.
 After relabeling, the `instance` label is set to the value of `__address__` by default if
 it was not set during relabeling. The `__scheme__` and `__metrics_path__` labels
-are set to the scheme and metrics path of the target respectively.
+are set to the scheme and metrics path of the target respectively. The `__param_<name>`
+label is set to the value of the first passed URL parameter called `<name>`.
 
 Additional labels prefixed with `__meta_` may be available during the
 relabeling phase. They are set by the service discovery mechanism that provided