Override and pass in additional grafana agent config keys (#161)

This permits template_file to be overridden, and for additional
variables to be passed through to the template file.

lib/prom_ex/config.ex

@@ -192,6 +192,8 @@ defmodule PromEx.Config do
 
       * `:template_file` - The full path to the template used to render the agent config file.
 
+      * all of these keys, and any additional ones, will be provided as EEx vars to the template file.
+
   * `:metrics_server` - This key contains the configuration information needed to run a standalone
     HTTP server powered by Cowboy. This server provides a lightweight solution to serving up PromEx
     metrics. Its configuration options are:
@@ -325,21 +327,22 @@ defmodule PromEx.Config do
 
   defp extract_opts_for_config(opts) do
     %{
-      scrape_interval: Keyword.get(opts, :scrape_interval, "15s"),
-      bearer_token: Keyword.get(opts, :bearer_token, "blank"),
-      log_level: Keyword.get(opts, :log_level, "error"),
-      agent_port: Keyword.get(opts, :agent_port, "4040"),
-      grpc_port: Keyword.get(opts, :grpc_port, "9095"),
-      job: Keyword.get(opts, :job, nil),
-      instance: Keyword.get(opts, :instance, nil),
+      scrape_interval: "15s",
+      bearer_token: "blank",
+      log_level: "error",
+      agent_port: "4040",
+      grpc_port: "9095",
+      job: nil,
+      instance: nil,
       prometheus_url: get_grafana_agent_config(opts, :prometheus_url),
       prometheus_username: get_grafana_agent_config(opts, :prometheus_username),
       prometheus_password: get_grafana_agent_config(opts, :prometheus_password),
-      metrics_server_path: Keyword.get(opts, :metrics_server_path, "/metrics"),
-      metrics_server_port: Keyword.get(opts, :metrics_server_port, 4000),
-      metrics_server_host: Keyword.get(opts, :metrics_server_host, "localhost"),
-      metrics_server_scheme: Keyword.get(opts, :metrics_server_scheme, :https)
+      metrics_server_path: "/metrics",
+      metrics_server_port: 4000,
+      metrics_server_host: "localhost",
+      metrics_server_scheme: :https
     }
+    |> Map.merge(Map.new(opts))
   end
 
   defp get_grafana_agent_config(grafana_agent_opts, config_key) do

test/prom_ex/config_test.exs

@@ -45,6 +45,77 @@ defmodule PromEx.ConfigTest do
              } = config
     end
 
+    test "should generate default grafana agent config" do
+      config =
+        Config.build(
+          grafana_agent: [
+            config_opts: [
+              prometheus_url: "https://prometheus",
+              prometheus_username: "prometheus-user",
+              prometheus_password: "prometheus-password"
+            ]
+          ]
+        )
+
+      assert %PromEx.Config{
+               grafana_agent_config: %{
+                 config_opts: %{
+                   prometheus_url: "https://prometheus",
+                   prometheus_username: "prometheus-user",
+                   prometheus_password: "prometheus-password",
+                   scrape_interval: "15s",
+                   bearer_token: "blank",
+                   log_level: "error",
+                   agent_port: "4040",
+                   grpc_port: "9095",
+                   job: nil,
+                   instance: nil,
+                   metrics_server_path: "/metrics",
+                   metrics_server_port: 4000,
+                   metrics_server_host: "localhost",
+                   metrics_server_scheme: :https
+                 }
+               }
+             } = config
+    end
+
+    test "should override existing keys, and pass additional keys to grafana agent config" do
+      config =
+        Config.build(
+          grafana_agent: [
+            config_opts: [
+              prometheus_url: "https://prometheus",
+              prometheus_username: "prometheus-user",
+              prometheus_password: "prometheus-password",
+              agent_port: "4141",
+              template_file: "path/to/template",
+              foo: "bar"
+            ]
+          ]
+        )
+
+      assert %PromEx.Config{
+               grafana_agent_config: %{
+                 config_opts: %{
+                   # required
+                   prometheus_url: "https://prometheus",
+                   prometheus_username: "prometheus-user",
+                   prometheus_password: "prometheus-password",
+
+                   # default
+                   scrape_interval: "15s",
+
+                   # override
+                   agent_port: "4141",
+
+                   # new keys
+                   template_file: "path/to/template",
+                   foo: "bar"
+                 }
+               }
+             } = config
+    end
+
     test "should raise an error if there are missing standalone metrics server fields" do
       assert_raise RuntimeError, "When configuring the PromEx metrics server, the :port key is required.", fn ->
         Config.build(metrics_server: [])