load integrations by default when possible

this way, one can document a less special migration path, and even
change integration dirs if so desired.

integration_tests/datadog_test.rb

@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
+require "ddtrace"
 require "test_helper"
 require "support/http_helpers"
-require "ddtrace"
 require "httpx/adapters/datadog"
 
 class DatadogTest < Minitest::Test

integration_tests/sentry_test.rb

@@ -1,15 +1,14 @@
 # frozen_string_literal: true
 
-require "logger"
+if RUBY_VERSION >= "2.4.0"
-require "stringio"
+  require "logger"
-require "test_helper"
+  require "stringio"
-require "support/http_helpers"
+  require "sentry-ruby"
-begin
+  require "test_helper"
+  require "support/http_helpers"
   require "httpx/adapters/sentry"
-rescue LoadError
-end
 
-class SentryTest < Minitest::Test
+  class SentryTest < Minitest::Test
     include HTTPHelpers
 
     DUMMY_DSN = "http://12345:67890@sentry.localdomain/sentry/42"
@@ -115,4 +114,5 @@ class SentryTest < Minitest::Test
     def origin
       "https://#{httpbin}"
     end
-end if RUBY_VERSION >= "2.4.0"
+  end
+end

integration_tests/webmock_test.rb

@@ -1,9 +1,9 @@
 # frozen_string_literal: true
 
-require "test_helper"
-require "support/http_helpers"
 require "webmock/minitest"
 require "httpx/adapters/webmock"
+require "test_helper"
+require "support/http_helpers"
 
 class WebmockTest < Minitest::Test
   include HTTPHelpers

lib/httpx.rb

@@ -67,3 +67,9 @@ end
 
 require "httpx/session"
 require "httpx/session_extensions"
+
+# load integrations when possible
+
+require "httpx/adapters/datadog" if defined?(DDTrace) || defined?(Datadog)
+require "httpx/adapters/sentry" if defined?(Sentry)
+require "httpx/adapters/webmock" if defined?(WebMock)