ensure ids for servo meters are consistent (Netflix#534)

Before the servo registry would return instances of ServoId
for Counter and Gauge types that would have additional tagging
and fail to match the id for the meter in the base registry
map. This would lead to null values when using the iterator
with a composite registry.

Fixes Netflix#530.

avoid eager singleton for optional binding (Netflix#528)

If set as eager it will force the default to be created
even if it is overridden.

update docs to refer to 0.61.0 (Netflix#519)

fix NPE for ServoGauge.measure() (Netflix#500)

Before it was incorrectly returning null instead of
a set of measurements.

update docs to recommend 0.59.0 (Netflix#498)

fix Netflix#483, ArrayTagSet.dedup overwrites first value (Netflix#484)

There was a bug in ArrayTagSet.dedup where if the source
list had multiple entries that are not duplicates, then
it would overwrite the value of the first entry with the
key of the second. This would lead to a shifted set of
tags and the last value is null. This causes a NPE when
attempting to compute the hashCode.

fix visibility for methods on TagsBuilder (Netflix#482)

They were package private which means anyone outside
of that package was unable to use the builder methods.

update docs for recent gauge changes (Netflix#476)

Avoid referring to deprecated methods in the docs and
describe using the new PolledMeter pattern.

update docs version to 0.57.1 (Netflix#446)

There is a bug with the publication of the log4j1
extension in 0.57.0. See Netflix#445 for details.

update version in docs to 0.57.0 (Netflix#443)