Improve structured logging

[clokep]

Building on #8588 there's a desire to improve the output of Synapse structured logging.

Currently a "standard" log line would be turned into JSON (this was a curl to /_matrix/client/r0/login to get the login flows, but other logs look similar):

2020-10-29 07:36:00,762 - synapse.access.http.8080 - 315 - INFO - GET-0 - - - 8080 - {None} Processed request: 0.000sec/-0.000sec (0.000sec, 0.000sec) (0.000sec/0.000sec/0) 285B 200 "GET /_matrix/client/r0/login HTTP/1.1" "curl/7.64.1" [0 dbevts]

This would transform to (note that whitespace was added for visibility, normally this would be a single line with no whitespaec between JSON separators):

{
    "log": "- - 8080 - {None} Processed request: 0.000sec/-0.000sec (0.000sec, 0.000sec) (0.000sec/0.000sec/0) 285B 200 \"GET /_matrix/client/r0/login HTTP/1.1\" \"curl/7.64.1\" [0 dbevts]",
    "namespace": "synapse.access.http.8080",
    "level": "INFO",
    "time": 1603971360.76,
    "request": "GET-0",
    "scope": null,
    "server_name": "localhost:8480"
}

A few improvements we'll want to make:

• The log line includes a formatted message, which makes it difficult to search in a structured way.
• Currently logging a message with additional fields requires including the fields twice (using the standard logging flow and the extra keyword to the logger).

[clokep]

To update this a bit:

• The code to create the structured logs has been cleaned up quite a bit so I think this issue is now in a spot to include additional info on a per-log line basis.
• We now include additional fields in the structed logs due to Record more information into structured logs #9654, which should make them more usable.

So the description here is still pretty valid (including the formatted log lines is not ideal), but much of the logging context is now included.

I'm no longer actively working on this so unassigning.