A guide to the request log lines format. (#8436)

This doc is short but a useful guide to what the request log lines mean.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Erik Johnston <erik@matrix.org>
Co-authored-by: Daniele Sluijters <daenney@users.noreply.github.com>

changelog.d/8436.doc

@@ -0,0 +1 @@
+Add a new guide to decoding request logs.

docs/SUMMARY.md

@@ -61,6 +61,7 @@
       - [Server Version](admin_api/version_api.md)
     - [Manhole](manhole.md)
     - [Monitoring](metrics-howto.md)
+    - [Request log format](usage/administration/request_log.md)
     - [Scripts]()
 
 # Development

docs/usage/administration/request_log.md

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+# Request log format
+
+HTTP request logs are written by synapse (see [`site.py`](../synapse/http/site.py) for details).
+
+See the following for how to decode the dense data available from the default logging configuration.
+
+```
+2020-10-01 12:00:00,000 - synapse.access.http.8008 - 311 - INFO - PUT-1000- 192.168.0.1 - 8008 - {another-matrix-server.com} Processed request: 0.100sec/-0.000sec (0.000sec, 0.000sec) (0.001sec/0.090sec/3) 11B !200 "PUT /_matrix/federation/v1/send/1600000000000 HTTP/1.1" "Synapse/1.20.1" [0 dbevts]
+-AAAAAAAAAAAAAAAAAAAAA-   -BBBBBBBBBBBBBBBBBBBBBB-   -C-   -DD-   -EEEEEE-  -FFFFFFFFF-   -GG-    -HHHHHHHHHHHHHHHHHHHHHHH-                     -IIIIII- -JJJJJJJ-  -KKKKKK-, -LLLLLL-  -MMMMMMM- -NNNNNN- O  -P- -QQ-  -RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR-   -SSSSSSSSSSSS-   -TTTTTT-
+```
+
+
+| Part  | Explanation | 
+| ----- | ------------ |
+| AAAA  | Timestamp request was logged (not recieved) |
+| BBBB  | Logger name (`synapse.access.(http\|https).<tag>`, where 'tag' is defined in the `listeners` config section, normally the port) |
+| CCCC  | Line number in code |
+| DDDD  | Log Level |
+| EEEE  | Request Identifier (This identifier is shared by related log lines)|
+| FFFF  | Source IP (Or X-Forwarded-For if enabled) |
+| GGGG  | Server Port |
+| HHHH  | Federated Server or Local User making request (blank if unauthenticated or not supplied) |
+| IIII  | Total Time to process the request |
+| JJJJ  | Time to send response over network once generated (this may be negative if the socket is closed before the response is generated)|
+| KKKK  | Userland CPU time |
+| LLLL  | System CPU time |
+| MMMM  | Total time waiting for a free DB connection from the pool across all parallel DB work from this request |
+| NNNN  | Total time waiting for response to DB queries across all parallel DB work from this request |
+| OOOO  | Count of DB transactions performed |
+| PPPP  | Response body size |
+| QQQQ  | Response status code (prefixed with ! if the socket was closed before the response was generated) |
+| RRRR  | Request |
+| SSSS  | User-agent |
+| TTTT  | Events fetched from DB to service this request (note that this does not include events fetched from the cache) |
+
+
+MMMM / NNNN can be greater than IIII if there are multiple slow database queries
+running in parallel.
+
+Some actions can result in multiple identical http requests, which will return
+the same data, but only the first request will report time/transactions in
+`KKKK`/`LLLL`/`MMMM`/`NNNN`/`OOOO` - the others will be awaiting the first query to return a
+response and will simultaneously return with the first request, but with very
+small processing times.