Add "request" to glossary

[SergeyKanzhelev]

Feedback:

Th term "Request" may be misread: overall transaction and component-specific. Should we update this terminology to distinguish these? For example: in the section on response header, the wording “Completed request” could be wrongly interpreted as the completion of the overall request, whereas it is for each participant while the overall request is still in progress.

[kalyanaj]

Thanks Sergey for filing this issue. Following up from our last discussion, I checked for the usage of the word "request" in the latest version of the trace context spec, and I do see it being used consistently to denote a request handled by an individual service/component as part of an overall logical operation. Agree it would still be good to call this out in the glossary though to make this terminology explicit.

That said, the introductory OTel documentation at https://opentelemetry.io/docs/concepts/data-sources/ uses a different terminology though where request is used to denote the logical operation flowing through a system, and my earlier interpretation of the word "request" in the trace context spec might have been influenced by that OTel documentation.