nixos/datadog: Don't recommend dd_url for sites, add proper option

Turns out, `dd_url` should only be used in proxy scenarios, not to point
datadog to their EU endpoint - `site` should be used for that.

The `dd_url` setting doesn't affect APM, Logs or Live Process intake
which have their own "*_dd_url" settings.

nixos/modules/services/monitoring/datadog-agent.nix

@@ -6,14 +6,15 @@ let
   cfg = config.services.datadog-agent;
 
   ddConf = {
-    dd_url              = cfg.ddUrl;
     skip_ssl_validation = false;
     confd_path          = "/etc/datadog-agent/conf.d";
     additional_checksd  = "/etc/datadog-agent/checks.d";
     use_dogstatsd       = true;
   }
   // optionalAttrs (cfg.logLevel != null) { log_level = cfg.logLevel; }
   // optionalAttrs (cfg.hostname != null) { inherit (cfg) hostname; }
+  // optionalAttrs (cfg.ddUrl != null) { dd_url = cfg.ddUrl; }
+  // optionalAttrs (cfg.site != null) { site = cfg.site; }
   // optionalAttrs (cfg.tags != null ) { tags = concatStringsSep ", " cfg.tags; }
   // optionalAttrs (cfg.enableLiveProcessCollection) { process_config = { enabled = "true"; }; }
   // optionalAttrs (cfg.enableTraceAgent) { apm_config = { enabled = true; }; }
@@ -79,14 +80,23 @@ in {
 
     ddUrl = mkOption {
       description = ''
-        Custom dd_url to configure the agent with.
-        Useful when you want to point datadog to another endpoint, either
-        because you need a proxy to send out data, or because you use their EU
-        endpoint.
+        Custom dd_url to configure the agent with. Useful if traffic to datadog
+        needs to go through a proxy.
+        Don't use this to point to another datadog site (EU) - use site instead.
       '';
-      default = "https://app.datadoghq.com";
-      example = "https://app.datadoghq.eu";
-      type = types.str;
+      default = null;
+      example = "http://haproxy.example.com:3834";
+      type = types.nullOr types.str;
+    };
+
+    site = mkOption {
+      description = ''
+        The datadog site to point the agent towards.
+        Set to datadoghq.eu to point it to their EU site.
+      '';
+      default = null;
+      example = "datadoghq.eu";
+      type = types.nullOr types.str;
     };
 
     tags = mkOption {