InstrumentedHandler: Remove duplicate calls to requests.update(...)

The timer is updated inside updateResponses(). This causes the count of this
timer to be double what it should be, and probably biased the times for the
exponentially decaying reservoirs.

Commit 33113e55cb fixed this same bug for activeRequests.
MeterSerializer.serialize had m1_rate and m15_rate transposed

There was a simple problem, where the MeterSerializer has the wrong
labels for m1_rate and m15_rate.
Updated Test Case for the MetricsModule meter serializer
loosen type requirements for JmxAttributeGauge constructor

Previously an MBeanServer was required to create a JmxAttributeGauge object
even though instances of JmxAttributeGauge only use methods from the
MBeanServer's super-interface, MBeanServerConnection. Relaxing this constraint
allows passing a JmxAttributeGauge any MBeanServerConnection (e.g. the
connection returned by JMXConnector.getMBeanServerConnection).
allow to specify a custom prefix for InstrumentedHandler for jetty9
Updated class comment to match new class name.
Merge pull request #462 from nirvdrum/fix_comment

Updated class comment to match new class name.
Merge pull request #457 from bconneen/master

Fixed MetricsModule's serlializer and unit test
Merge pull request #458 from jfocht/master

loosen type requirements for JmxAttributeGauge constructor
Merge pull request #460 from 50onRed/master

allow to specify a custom prefix for InstrumentedHandler for jetty9
Merge pull request #445 from evanj/master

InstrumentedHandler: Remove duplicate calls to requests.update(...)