Breaking Changes

Remove previously deprecated Continuation.close() methods and make Continuation static #1419
- Use Continuation.cancel() instead

Improvements

[Performance] avoid materialising meta (baggage + tags) during serialisation #1472
[Performance] move batching to trace processor to ease memory buffer reuse #1454
[Community] Adding an option to manually disable Kafka headers #1448 (Thanks @shields478 for the contribution!)
Add version specific names to lettuce instrumentation to allow disabling only a specific version #1456
[Performance] Use byte[] keys in FormatWriter to save on hashmap lookups when the keys are constant #1455
[Performance] Reduce copy/allocation in trace serialization pipeline #1437, #1471
[Performance] Wrap log statements using varargs to avoid object allocation. #1466
Add Grizzly-HTTP, Grizzly HTTP Client, and ning Http Client instrumentations #1365
- These instrumentations are disabled by default. Enable using:
  - System Properties:
    -Ddd.integration.grizzly-filterchain.enabled=true
    -Ddd.integration.grizzly-client.enabled=true
    -Ddd.integration.ning.enabled=true
  - Environment Variables:
    DD_INTEGRATION_GRIZZLY_FILTERCHAIN_ENABLED=true
    DD_INTEGRATION_GRIZZLY_CLIENT_ENABLED=true
    DD_INTEGRATION_NING_ENABLED=true