add support to delegate to spectator

[brharrington]

This is a first stab at adding a hook that will allow updates that go to standard Servo types to be forwarded to a Spectator registry. In most cases the updates will be sent as they happen, e.g., the basic counter will increment the spectator counter internally. This avoids delays or other data quality issues.

Because so much of Servo is based on static, the main integration point is a static reference in SpectatorContext. The default registry is a noop implementation so it must be set as early as possible in the application setup.

To minimize the compatibility issues the existing Servo implementations were changed as little as possible. Concerns:

• Custom monitor implementations that get registered will currently get ignored. Will look into that later.
• User defined functions registered as gauges will now get called more frequently. This might cause load issues if they perform expensive operations inline.

[brharrington]

Most recent test failure is:

Gradle suite > Gradle test > com.netflix.servo.monitor.StatsMonitorTest.testExpiration FAILED
    java.lang.AssertionError at StatsMonitorTest.java:42

Only on jdk8 build, looks like a flakey test to me.

[brharrington]

Still need to add tests for:

• [x] Custom monitors
• [x] BucketTimer
• [x] ContextualCounter
• [x] ContextualTimer
• [x] StatsTimer
• [x] PeakRateCounter
• [x] MinGauge