Swift loggers encapsulate a StatsdClient that is typically initialised
with a prefix, equal to the logger name (e.g. 'proxy_server'), that is
prepended to metrics names. The proxy server would previously mutate
its logger's prefix, using its set_statsd_prefix method, each time a
controller was instantiated, extending it with the controller type
(e.g. changing the prefix 'proxy_server.object'). As a result, when an
object request spawned container subrequests, for example, the statsd
client would be left with a 'proxy_server.container' prefix part for
subsequent object request related metrics.
The proxy server logger is now wrapped with a new
MetricsPrefixLoggerAdapter each time a controller is instantiated, and
the adapter applies the correct prefix for the controller type for the
lifetime of the controller.
Change-Id: I0522b1953722ca96021a0002cf93432b973ce626
6942b25cc1