Add JSON log format to logging configuration (#2982)

Now patroni can be configured as bellow to log in json format.

```yaml
log:
  type: json
  format:
    - asctime: '@timestamp'
    - levelname: level
    - message
    - module
    - name: logger_name
  static_fields:
    app: patroni
```

This config produce this log:

```json
{
  "@timestamp": "2023-12-14 19:51:24,872",
  "level": "INFO",
  "message": "Lock owner: None; I am postgresql1",
  "module": "ha",
  "app": "patroni",
  "logger_name": "patroni.ha"
}
```

docs/ENVIRONMENT.rst

@@ -14,10 +14,18 @@ Global/Universal
 
 Log
 ---
+-  **PATRONI\_LOG\_TYPE**: sets the format of logs. Can be either **plain** or **json**. To use **json** format, you must have the :ref:`jsonlogger <extras>` installed. The default value is **plain**.
 -  **PATRONI\_LOG\_LEVEL**: sets the general logging level. Default value is **INFO** (see `the docs for Python logging <https://docs.python.org/3.6/library/logging.html#levels>`_)
 -  **PATRONI\_LOG\_TRACEBACK\_LEVEL**: sets the level where tracebacks will be visible. Default value is **ERROR**. Set it to **DEBUG** if you want to see tracebacks only if you enable **PATRONI\_LOG\_LEVEL=DEBUG**.
--  **PATRONI\_LOG\_FORMAT**: sets the log formatting string. Default value is **%(asctime)s %(levelname)s: %(message)s** (see `the LogRecord attributes <https://docs.python.org/3.6/library/logging.html#logrecord-attributes>`_)
+-  **PATRONI\_LOG\_FORMAT**: sets the log formatting string. If the log type is **plain**, the log format should be a string.
+   Refer to `the LogRecord attributes <https://docs.python.org/3.6/library/logging.html#logrecord-attributes>`_ for
+   available attributes. If the log type is **json**, the log format can be a list in addition to a string. Each list
+   item should correspond to LogRecord attributes. Be cautious that only the field name is required, and the **%(**
+   and **)** should be omitted. If you wish to print a log field with a different key name, use a dictionary where
+   the dictionary key is the log field, and the value is the name of the field you want to be printed in the log.
+   Default value is **%(asctime)s %(levelname)s: %(message)s**
 -  **PATRONI\_LOG\_DATEFORMAT**: sets the datetime formatting string. (see the `formatTime() documentation <https://docs.python.org/3.6/library/logging.html#logging.Formatter.formatTime>`_)
+-  **PATRONI\_LOG\_STATIC\_FIELDS**: add additional fields to the log. This option is only available when the log type is set to **json**. Example ``PATRONI_LOG_STATIC_FIELDS="{app: patroni}"``
 -  **PATRONI\_LOG\_MAX\_QUEUE\_SIZE**: Patroni is using two-step logging. Log records are written into the in-memory queue and there is a separate thread which pulls them from the queue and writes to stderr or file. The maximum size of the internal queue is limited by default by **1000** records, which is enough to keep logs for the past 1h20m.
 -  **PATRONI\_LOG\_DIR**: Directory to write application logs to. The directory must exist and be writable by the user executing Patroni. If you set this env variable, the application will retain 4 25MB logs by default. You can tune those retention values with `PATRONI_LOG_FILE_NUM` and `PATRONI_LOG_FILE_SIZE` (see below).
 -  **PATRONI\_LOG\_FILE\_NUM**: The number of application logs to retain.

docs/installation.rst

@@ -60,6 +60,8 @@ raft
     `pysyncobj` module in order to use python Raft implementation as DCS
 aws
     `boto3` in order to use AWS callbacks
+jsonlogger
+    `python-json-logger` module in order to enable :ref:`logging <log_settings>` in json format
 all
     all of the above (except psycopg family)
 psycopg

docs/yaml_configuration.rst

@@ -11,12 +11,22 @@ Global/Universal
 -  **namespace**: path within the configuration store where Patroni will keep information about the cluster. Default value: "/service"
 -  **scope**: cluster name
 
+.. _log_settings:
+
 Log
 ---
+-  **type**: sets the format of logs. Can be either **plain** or **json**. To use **json** format, you must have the :ref:`jsonlogger <extras>` installed. The default value is **plain**.
 -  **level**: sets the general logging level. Default value is **INFO** (see `the docs for Python logging <https://docs.python.org/3.6/library/logging.html#levels>`_)
 -  **traceback\_level**: sets the level where tracebacks will be visible. Default value is **ERROR**. Set it to **DEBUG** if you want to see tracebacks only if you enable **log.level=DEBUG**.
--  **format**: sets the log formatting string. Default value is **%(asctime)s %(levelname)s: %(message)s** (see `the LogRecord attributes <https://docs.python.org/3.6/library/logging.html#logrecord-attributes>`_)
+-  **format**: sets the log formatting string. If the log type is **plain**, the log format should be a string. Refer to
+   `the LogRecord attributes <https://docs.python.org/3.6/library/logging.html#logrecord-attributes>`_ for
+   available attributes. If the log type is **json**, the log format can be a list in addition to a string. Each list
+   item should correspond to LogRecord attributes. Be cautious that only the field name is required, and the **%(**
+   and **)** should be omitted. If you wish to print a log field with a different key name, use a dictionary where
+   the dictionary key is the log field, and the value is the name of the field you want to be printed in the log.
+   Default value is **%(asctime)s %(levelname)s: %(message)s**
 -  **dateformat**: sets the datetime formatting string. (see the `formatTime() documentation <https://docs.python.org/3.6/library/logging.html#logging.Formatter.formatTime>`_)
+-  **static_fields**: add additional fields to the log. This option is only available when the log type is set to **json**.
 -  **max\_queue\_size**: Patroni is using two-step logging. Log records are written into the in-memory queue and there is a separate thread which pulls them from the queue and writes to stderr or file. The maximum size of the internal queue is limited by default by **1000** records, which is enough to keep logs for the past 1h20m.
 -  **dir**: Directory to write application logs to. The directory must exist and be writable by the user executing Patroni. If you set this value, the application will retain 4 25MB logs by default. You can tune those retention values with `file_num` and `file_size` (see below).
 -  **file\_num**: The number of application logs to retain.
@@ -26,6 +36,20 @@ Log
    -  **patroni.postmaster: WARNING**
    -  **urllib3: DEBUG**
 
+Here is an example of how to config patroni to log in json format.
+
+.. code:: YAML
+
+   log:
+      type: json
+      format:
+         - message
+         - module
+         - asctime: '@timestamp'
+         - levelname: level
+      static_fields:
+         app: patroni
+
 .. _bootstrap_settings:
 
 Bootstrap configuration