Require the main prometheus_exporter module in client and server modules

The main module includes some consts and defaults, which both client and server require to work.
In the README, it is suggested to treat client and server modules as entry-points,
but without requiring the main module they won't work.

In this commit, I'm adding a require and tests to show the problem.

Author: Pawel Rutkowski (Work)

lib/prometheus_exporter/client.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require_relative "../prometheus_exporter"
 require "socket"
 require "logger"
 

lib/prometheus_exporter/server.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require_relative "../prometheus_exporter"
 require_relative "metric"
 require_relative "server/type_collector"
 require_relative "server/web_collector"

test/client_independent_loading_test.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+# This test verifies that the client can be loaded independently
+# without requiring the main prometheus_exporter module first.
+#
+# DO NOT require test_helper here, as it pre-loads the main module.
+
+require "minitest/autorun"
+
+class ClientIndependentLoadingTest < Minitest::Test
+  def test_client_has_constants_when_loaded_independently
+    require_relative "../lib/prometheus_exporter/client"
+
+    assert_equal 9394, PrometheusExporter::DEFAULT_PORT
+    assert_equal "localhost", PrometheusExporter::DEFAULT_BIND_ADDRESS
+    assert_equal 2, PrometheusExporter::DEFAULT_TIMEOUT
+  end
+end

test/server_independent_loading_test.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+# This test verifies that the server can be loaded independently
+# without requiring the main prometheus_exporter module first.
+#
+# DO NOT require test_helper here, as it pre-loads the main module.
+
+require "minitest/autorun"
+
+class ServerIndependentLoadingTest < Minitest::Test
+  def test_server_has_constants_when_loaded_independently
+    require_relative "../lib/prometheus_exporter/server"
+
+    assert_equal 9394, PrometheusExporter::DEFAULT_PORT
+    assert_equal "localhost", PrometheusExporter::DEFAULT_BIND_ADDRESS
+    assert_equal 2, PrometheusExporter::DEFAULT_TIMEOUT
+    assert_equal "ruby_", PrometheusExporter::DEFAULT_PREFIX
+    assert_equal "Prometheus Exporter", PrometheusExporter::DEFAULT_REALM
+  end
+end