tracing: allow constant field names in macros

[hawkw]

This change was initially contributed by @wyfo in PR #2617 --- I've just opened a new PR after updating that branch to track the latest changes on master.

Motivation

I've found myself in the case where I wanted to have customized event field name for different trait implementations. In fact, these implementations are completely unrelated (in separate applications), so, in this use case, I find more readable to have foo="some_id" and bar=16 instead of resource="foo" value="some_id" and resource=bar value=16

Because events only accept identifier or literal as field name, this is quite cumbersome/impossible to do. A simple solution could be to make events accept constant expression too; in my use case, I could then add a associated constant to my trait.

Solution

This PR proposes a new syntax for using constant field names:

tracing::debug!({ CONSTANT_EXPR } = "foo");

This is the same syntax than constant expression, so it should be quite intuitive.

To avoid constant expression names conflict, internal variables of macro expansion have been prefixed with __, e.g. __CALLSITE.

Closes #2617