dd-agent: Add default config files of dd-agent and auto_conf dir

to /etc/dd-agent/conf.d by default, and make sure
/etc/dd-agent/conf.d is used.

Before NixOS 17.03, we were using dd-agent 5.5.X which
used configuration from /etc/dd-agent/conf.d

In NixOS 17.03 the default conf.d location is first used relative,
meaning that $out/agent/conf.d was used without NixOS overrides.

This change implements similar functionality as PR #25288, without
breaking backwards compatibility.

nixos/modules/module-list.nix

@@ -314,7 +314,7 @@
   ./services/monitoring/cadvisor.nix
   ./services/monitoring/collectd.nix
   ./services/monitoring/das_watchdog.nix
-  ./services/monitoring/dd-agent.nix
+  ./services/monitoring/dd-agent/dd-agent.nix
   ./services/monitoring/grafana.nix
   ./services/monitoring/graphite.nix
   ./services/monitoring/hdaps.nix

nixos/modules/services/monitoring/dd-agent/dd-agent-defaults.nix

@@ -0,0 +1,8 @@
+# Generated using update-dd-agent-default, please re-run after updating dd-agent. DO NOT EDIT MANUALLY.
+[
+  "auto_conf"
+  "agent_metrics.yaml.default"
+  "disk.yaml.default"
+  "network.yaml.default"
+  "ntp.yaml.default"
+]

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

@@ -75,7 +75,12 @@ let
   jmxConfig = pkgs.writeText "jmx.yaml" cfg.jmxConfig;
   
   etcfiles =
-    [ { source = ddConf;
+    let
+      defaultConfd = import ./dd-agent-defaults.nix;
+    in (map (f: { source = "${pkgs.dd-agent}/agent/conf.d-system/${f}";
+                  target = "dd-agent/conf.d/${f}";
+                }) defaultConfd) ++ [
+      { source = ddConf;
         target = "dd-agent/datadog.conf";
       }
       { source = diskConfig;

nixos/modules/services/monitoring/dd-agent/update-dd-agent-defaults

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+dd=$(nix-build --no-out-link -A dd-agent ../../../..)
+echo '# Generated using update-dd-agent-default, please re-run after updating dd-agent. DO NOT EDIT MANUALLY.' > dd-agent-defaults.nix
+echo '[' >> dd-agent-defaults.nix
+echo '  "auto_conf"' >> dd-agent-defaults.nix
+for f in $(find $dd/agent/conf.d-system -maxdepth 1 -type f | grep -v '\.example' | sort); do
+  echo "  \"$(basename $f)\"" >> dd-agent-defaults.nix
+done
+echo ']' >> dd-agent-defaults.nix

pkgs/tools/networking/dd-agent/default.nix

@@ -64,6 +64,9 @@ in stdenv.mkDerivation rec {
     ln -s $out/agent/dogstatsd.py $out/bin/dogstatsd
     ln -s $out/agent/ddagent.py $out/bin/dd-forwarder
 
+    # Move out default conf.d so that /etc/dd-agent/conf.d is used
+    mv $out/agent/conf.d $out/agent/conf.d-system
+
     cat > $out/bin/dd-jmxfetch <<EOF
     #!/usr/bin/env bash
     exec ${python}/bin/python $out/agent/jmxfetch.py $@