Implement a structured logging output system. (#5680)

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+# Structured Logging
+
+A structured logging system can be useful when your logs are destined for a machine to parse and process. By maintaining its machine-readable characteristics, it enables more efficient searching and aggregations when consumed by software such as the "ELK stack".
+
+Synapse's structured logging system is configured via the file that Synapse's `log_config` config option points to. The file must be YAML and contain `structured: true`. It must contain a list of "drains" (places where logs go to).
+
+A structured logging configuration looks similar to the following:
+
+```yaml
+structured: true
+
+loggers:
+    synapse:
+        level: INFO
+    synapse.storage.SQL:
+        level: WARNING
+
+drains:
+    console:
+        type: console
+        location: stdout
+    file:
+        type: file_json
+        location: homeserver.log
+```
+
+The above logging config will set Synapse as 'INFO' logging level by default, with the SQL layer at 'WARNING', and will have two logging drains (to the console and to a file, stored as JSON).
+
+## Drain Types
+
+Drain types can be specified by the `type` key.
+
+### `console`
+
+Outputs human-readable logs to the console.
+
+Arguments:
+
+- `location`: Either `stdout` or `stderr`.
+
+### `console_json`
+
+Outputs machine-readable JSON logs to the console.
+
+Arguments:
+
+- `location`: Either `stdout` or `stderr`.
+
+### `console_json_terse`
+
+Outputs machine-readable JSON logs to the console, separated by newlines. This
+format is not designed to be read and re-formatted into human-readable text, but
+is optimal for a logging aggregation system.
+
+Arguments:
+
+- `location`: Either `stdout` or `stderr`.
+
+### `file`
+
+Outputs human-readable logs to a file.
+
+Arguments:
+
+- `location`: An absolute path to the file to log to.
+
+### `file_json`
+
+Outputs machine-readable logs to a file.
+
+Arguments:
+
+- `location`: An absolute path to the file to log to.
+
+### `network_json_terse`
+
+Delivers machine-readable JSON logs to a log aggregator over TCP. This is
+compatible with LogStash's TCP input with the codec set to `json_lines`.
+
+Arguments:
+
+- `host`: Hostname or IP address of the log aggregator.
+- `port`: Numerical port to contact on the host.
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