docs: Create a Glossay for types implementing Collect and Subscribers

[davidbarsky]

Feature Request

Crates

This applies to all crates.

Motivation

Some folks have expressed confusion over the new naming scheme for types implementing Collect and Subscribe (#1022 (comment)).

Proposal

Our current styleguide proposal is the following:

• Collect and Subscribe, with code formatting, refer to the traits.
• Collector and subscriber, with no code formatting or special casing, refer to types implementing the Collect and Subscribe traits.
• Types implementing Collect and Subscribe should be suffixed with Collector and Subscriber, respectively.

This glossary should be placed in a prominent location in the tracing documentation.

Alternatives

Don't do this, but this seems to be an unabiguious win.

[davidbarsky]

cc: @jyn514