v0.16.0 Release

• Add APIs to register gRPC client and server views separately.
• Add an API MeasureMap.putAttachment() for recording exemplars.
• Add Exemplar class and an API to get Exemplar list to DistributionData.
• Improve the styling of Rpcz, Statsz, Tracez, and Traceconfigz pages.
• Add an artifact opencensus-contrib-exemplar-util that has helper utilities
  on recording exemplars.
• Reduce the default limit on Links per Span to 32 (was 128 before).
• Add Spring support for @Traced annotation and java.sql.PreparedStatements
  tracing.
• Allow custom prefix for Stackdriver metrics in StackdriverStatsConfiguration.
• Add support to handle the Tracestate in the SpanContext.
• Remove global synchronization from the get current stats state.
• Add get/from{Byte} methods on TraceOptions and deprecate get/from{Bytes}.
• Add an API to StackdriverTraceConfiguration to allow setting a
  TraceServiceStub instance to be used for export RPC calls.
• Add an experimental artifact, opencensus-contrib-log-correlation-log4j2, for
  adding tracing data to Log4j 2 LogEvents.