nixos/prometheus: add scrape_configs.honor_labels

nixos/modules/services/monitoring/prometheus/default.nix

@@ -100,6 +100,29 @@ let
           The HTTP resource path on which to fetch metrics from targets.
         '';
       };
+      honor_labels = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Controls how Prometheus handles conflicts between labels
+          that are already present in scraped data and labels that
+          Prometheus would attach server-side ("job" and "instance"
+          labels, manually configured target labels, and labels
+          generated by service discovery implementations).
+
+          If honor_labels is set to "true", label conflicts are
+          resolved by keeping label values from the scraped data and
+          ignoring the conflicting server-side labels.
+
+          If honor_labels is set to "false", label conflicts are
+          resolved by renaming conflicting labels in the scraped data
+          to "exported_<original-label>" (for example
+          "exported_instance", "exported_job") and then attaching
+          server-side labels. This is useful for use cases such as
+          federation, where all labels specified in the target should
+          be preserved.
+        '';
+      };
       scheme = mkOption {
         type = types.enum ["http" "https"];
         default = "http";