Document the log manipulation facilities.

docs/how-to/change-log-level.rst

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+======================
+Changing the log level
+======================
+
+By default, the MicroCeph daemon runs with the log level set to DEBUG. While that is the desirable
+behaviour for a good number of use cases, there are instances when this level is far too high -
+for example, embedded devices where storage is much more limited. For these reasons, the MicroCeph
+daemon exposes a way to both get and set the log level.
+
+Configuring the log level
+-------------------------
+
+MicroCeph includes the command ``log``, with the sub-commands ``set-level`` and ``get-level``. When setting, we support both string and integer formats for the log level. For example:
+
+.. code-block:: none
+
+   sudo microceph log set-level warning
+   sudo microceph log set-level 3
+
+Both commands are equivalent. The mapping from integer to string can be consulted by querying the
+help for the ``set-level`` sub-command. Note that any changes made to the log level take effect
+immediately, and need no restarts.
+
+On the other hand, the ``get-level`` sub-command takes no arguments and returns an integer level only.
+Any value returned by ``get-level`` can be used for ``set-level``.
+
+For example, after setting the level as shown in the example, we can verify in this way:
+
+.. code-block:: none
+
+   sudo microceph log get-level
+   3
+
+