Make code blocks real code blocks in metrics docs

Makes them an RST block quote and 4 space indents instead of 2.

Change-Id: If2b727ac431e70d714f3df0c622c5516901f3393

doc/source/metrics.rst

@@ -21,7 +21,7 @@ Using a context manager
 Using the context manager is the recommended way for sending metrics that time
 or count sections of code. However, given that you cannot cache the
 MetricsLogger, you have to explicitly call get_metrics_logger() from
-ironic-lib every time. For example:
+ironic-lib every time. For example::
 
   from ironic_lib import metrics_utils
 
@@ -39,7 +39,7 @@ Explicitly sending metrics
 A feature that may be particularly helpful for deployers writing custom
 HardwareManagers is the ability to explicitly send metrics. As an example,
 you could add a cleaning step which would retrieve metrics about a device and
-ship them using the provided metrics library. For example:
+ship them using the provided metrics library. For example::
 
   from ironic_lib import metrics_utils