Indicate what endpoint came back with a JSON response we were unable to parse (#14097)

**Before:**
```
WARNING - POST-11 - Unable to parse JSON: Expecting value: line 1 column 1 (char 0) (b'')
```

**After:**
```
WARNING - POST-11 - Unable to parse JSON from POST /_matrix/client/v3/join/%21ZlmJtelqFroDRJYZaq:hs1?server_name=hs1 response: Expecting value: line 1 column 1 (char 0) (b'')
```

---

It's possible to figure out which endpoint these warnings were coming from before but you had to follow the request ID `POST-11` to the log line that says `Completed request [...]`. Including this key information next to the JSON parsing error makes it much easier to reason whether it matters or not.

```
2022-09-29T08:23:25.7875506Z synapse_main | 2022-09-29 08:21:10,336 - synapse.http.matrixfederationclient - 299 - INFO - POST-11 - {GET-O-13} [hs1] Completed request: 200 OK in 0.53 secs, got 450 bytes - GET matrix://hs1/_matrix/federation/v1/make_join/%21ohtKoQiXlPePSycXwp%3Ahs1/%40charlie%3Ahs2?ver=1&ver=2&ver=3&ver=4&ver=5&ver=6&ver=org.matrix.msc2176&ver=7&ver=8&ver=9&ver=org.matrix.msc3787&ver=10&ver=org.matrix.msc2716v4
```


---

As a note, having no `body` is normal for the `/join` endpoint and it can handle it.

https://github.com/matrix-org/synapse/blob/0c853e09709d52783efd37060ed9e8f55a4fc704/synapse/rest/client/room.py#L398-L403

Alternatively we could remove these extra logs but they are probably more usually helpful to figure out what went wrong.

3 files changed, 12 insertions, 2 deletions

diff --git a/changelog.d/14097.misc b/changelog.d/14097.misc
new file mode 100644
index 0000000000..8392448c4d
--- /dev/null
+++ b/changelog.d/14097.misc
@@ -0,0 +1 @@
+Indicate what endpoint came back with a JSON response we were unable to parse.
diff --git a/synapse/http/servlet.py b/synapse/http/servlet.py
index 80acbdcf3c..dead02cd5c 100644
--- a/synapse/http/servlet.py
+++ b/synapse/http/servlet.py
@@ -35,6 +35,7 @@ from typing_extensions import Literal
 from twisted.web.server import Request
 
 from synapse.api.errors import Codes, SynapseError
+from synapse.http import redact_uri
 from synapse.http.server import HttpServer
 from synapse.types import JsonDict, RoomAlias, RoomID
 from synapse.util import json_decoder
@@ -664,7 +665,13 @@ def parse_json_value_from_request(
     try:
         content = json_decoder.decode(content_bytes.decode("utf-8"))
     except Exception as e:
-        logger.warning("Unable to parse JSON: %s (%s)", e, content_bytes)
+        logger.warning(
+            "Unable to parse JSON from %s %s response: %s (%s)",
+            request.method.decode("ascii", errors="replace"),
+            redact_uri(request.uri.decode("ascii", errors="replace")),
+            e,
+            content_bytes,
+        )
         raise SynapseError(
             HTTPStatus.BAD_REQUEST, "Content not JSON.", errcode=Codes.NOT_JSON
         )
diff --git a/tests/http/test_servlet.py b/tests/http/test_servlet.py
index 3cbca0f5a3..46166292fe 100644
--- a/tests/http/test_servlet.py
+++ b/tests/http/test_servlet.py
@@ -35,11 +35,13 @@ from tests.http.server._base import test_disconnect
 
 def make_request(content):
     """Make an object that acts enough like a request."""
-    request = Mock(spec=["content"])
+    request = Mock(spec=["method", "uri", "content"])
 
     if isinstance(content, dict):
         content = json.dumps(content).encode("utf8")
 
+    request.method = bytes("STUB_METHOD", "ascii")
+    request.uri = bytes("/test_stub_uri", "ascii")
     request.content = BytesIO(content)
     return request