nixos/prometheus: add configText option for alertmanager

The reason being less mental overhead when reading upstream
documentation. Examples can be pasted right into the configuration
instead of translating to Nix attrset first.

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

@@ -5,6 +5,10 @@ with lib;
 let
   cfg = config.services.prometheus.alertmanager;
   mkConfigFile = pkgs.writeText "alertmanager.yml" (builtins.toJSON cfg.configuration);
+  alertmanagerYml =
+    if cfg.configText != null then
+      pkgs.writeText "alertmanager.yml" cfg.configText
+    else mkConfigFile;
 in {
   options = {
     services.prometheus.alertmanager = {
@@ -34,6 +38,17 @@ in {
         '';
       };
 
+      configText = mkOption {
+        type = types.nullOr types.lines;
+        default = null;
+        description = ''
+          Alertmanager configuration as YAML text. If non-null, this option
+          defines the text that is written to alertmanager.yml. If null, the
+          contents of alertmanager.yml is generated from the structured config
+          options.
+        '';
+      };
+
       logFormat = mkOption {
         type = types.nullOr types.str;
         default = null;
@@ -96,7 +111,7 @@ in {
       after    = [ "network.target" ];
       script = ''
         ${pkgs.prometheus-alertmanager.bin}/bin/alertmanager \
-        -config.file ${mkConfigFile} \
+        -config.file ${alertmanagerYml} \
         -web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
         -log.level ${cfg.logLevel} \
         ${optionalString (cfg.webExternalUrl != null) ''-web.external-url ${cfg.webExternalUrl} \''}