Autorelease 6.21.0

* improvement: add a Parent-Request-Id header

Autorelease 6.20.0

* improvement: broaden the requestTrace attachment accessor

Autorelease 6.19.0

* improvement: Provide an API to access the detached trace associated
  with an exchange

Remove references to AsyncSlf4jSpanObserver from README (#1063)

Autorelease 6.17.0

[skip ci]

Autorelease 6.16.0

[skip ci]

Autorelease 6.15.0

Autorelease 6.14.0

Autorelease 6.13.0

Autorelease 6.12.0