Implement the /threads endpoint from MSC3856.
This is currently unstable and behind an experimental configuration
flag.
It includes a background update to backfill data, results from
the /threads endpoint will be partial until that finishes.
**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.
0c853e0970/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.
Fixes two related bugs:
* No edit information was bundled for events which aren't `m.room.message`.
* `m.new_content` was not applied for those events.
Attempt to parse any valid information from an oEmbed response
(instead of bailing at the first unexpected data). This should allow
for more partial oEmbed data to be returned, resulting in better /
more URL previews, even if those URL previews are only partial.
==============================
Please note that legacy Prometheus metric names are now deprecated and will be removed in Synapse 1.73.0.
Server administrators should update their dashboards and alerting rules to avoid using the deprecated metric names.
See the [upgrade notes](https://matrix-org.github.io/synapse/v1.69/upgrade.html#upgrading-to-v1690) for more details.
Deprecations and Removals
-------------------------
- Deprecate the `generate_short_term_login_token` method in favor of an async `create_login_token` method in the Module API. ([\#13842](https://github.com/matrix-org/synapse/issues/13842))
Internal Changes
----------------
- Ensure Synapse v1.69 works with upcoming database changes in v1.70. ([\#14045](https://github.com/matrix-org/synapse/issues/14045))
- Fix a bug introduced in Synapse v1.68.0 where messages could not be sent in rooms with non-integer `notifications` power level. ([\#14073](https://github.com/matrix-org/synapse/issues/14073))
- Temporarily pin build-system requirements to workaround an incompatibility with poetry-core 1.3.0. This will be reverted before the v1.69.0 release proper, see [\#14079](https://github.com/matrix-org/synapse/issues/14079). ([\#14080](https://github.com/matrix-org/synapse/issues/14080))
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Merge tag 'v1.69.0rc2' into develop
Synapse 1.69.0rc2 (2022-10-06)
==============================
Please note that legacy Prometheus metric names are now deprecated and will be removed in Synapse 1.73.0.
Server administrators should update their dashboards and alerting rules to avoid using the deprecated metric names.
See the [upgrade notes](https://matrix-org.github.io/synapse/v1.69/upgrade.html#upgrading-to-v1690) for more details.
Deprecations and Removals
-------------------------
- Deprecate the `generate_short_term_login_token` method in favor of an async `create_login_token` method in the Module API. ([\#13842](https://github.com/matrix-org/synapse/issues/13842))
Internal Changes
----------------
- Ensure Synapse v1.69 works with upcoming database changes in v1.70. ([\#14045](https://github.com/matrix-org/synapse/issues/14045))
- Fix a bug introduced in Synapse v1.68.0 where messages could not be sent in rooms with non-integer `notifications` power level. ([\#14073](https://github.com/matrix-org/synapse/issues/14073))
- Temporarily pin build-system requirements to workaround an incompatibility with poetry-core 1.3.0. This will be reverted before the v1.69.0 release proper, see [\#14079](https://github.com/matrix-org/synapse/issues/14079). ([\#14080](https://github.com/matrix-org/synapse/issues/14080))
