Use Retry-After header in addition to retry_after_ms

[sents]

Description:

When exceeding the rate limit synapse sends a 429, M_LIMIT_EXCEEDED response with the json-attribute retry_after_ms. There seems to be a standard response header for this: Retry-After
Using this header in addition to the retry_after_ms attribute would make it easier to handle the retries in common http libraries.

[anoadragon453]

Nice idea! Though this is more of an issue for the Matrix Specification repo than Synapse. Once it's cleared in the spec it can be added to Synapse :)

[sents]

Ok, then I will open this issue in the spec repo. Thank you!