A HTTP message builder on which header values can be written and a complete HTTP message can be build from.
Implementations will typically wrap a HTTP message model from an instrumented framework and either accumulate
all header writes until a call to to .build() is made and a new HTTP message is constructed merging the previous
and accumulated headers (on immutable HTTP models) or directly write the headers on the underlying HTTP message
(on mutable HTTP models).
A HTTP message builder on which header values can be written and a complete HTTP message can be build from. Implementations will typically wrap a HTTP message model from an instrumented framework and either accumulate all header writes until a call to to .build() is made and a new HTTP message is constructed merging the previous and accumulated headers (on immutable HTTP models) or directly write the headers on the underlying HTTP message (on mutable HTTP models).