doc: add howto for the new expect() method

doc/howto.rst

@@ -238,6 +238,17 @@ these.
     the server.
 
 
+Using custom request matcher
+----------------------------
+In the case when you want to extend or modify the request matcher in
+*pytest-httpserrver*, then you can use your own request matcher.
+
+Example:
+
+.. literalinclude :: ../tests/examples/test_howto_custom_request_matcher.py
+   :language: python
+
+
 Customizing host and port
 -------------------------
 

tests/examples/test_howto_custom_request_matcher.py

@@ -0,0 +1,31 @@
+import requests
+from werkzeug import Request
+
+from pytest_httpserver import HTTPServer
+from pytest_httpserver import RequestMatcher
+
+
+class MyMatcher(RequestMatcher):
+    def match(self, request: Request) -> bool:
+        match = super().match(request)
+        if not match:  # existing parameters didn't match -> return with False
+            return match
+
+        # match the json's "value" key
+        # if it it can be divided by 2, it returns True, False otherwise
+        json = request.json
+        if isinstance(json, dict) and isinstance(json.get("value"), int):
+            return json["value"] % 2 == 0
+
+        return False
+
+
+def test_custom_request_matcher(httpserver: HTTPServer):
+    httpserver.expect(MyMatcher("/foo")).respond_with_data("OK")
+
+    resp = requests.post(httpserver.url_for("/foo"), json={"value": 42})
+    resp.raise_for_status()
+    assert resp.text == "OK"
+
+    resp = requests.post(httpserver.url_for("/foo"), json={"value": 43})
+    assert resp.status_code == 500