tracing: allow constant field names in macros (#2617)

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.

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

Co-authored-by: Joseph Perez <joseph.perez@numberly.com>
Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tracing/src/lib.rs

@@ -319,6 +319,19 @@
 //! # }
 //!```
 //!
+//! Constant expressions can also be used as field names. Constants
+//! must be enclosed in curly braces (`{}`) to indicate that the *value*
+//! of the constant is to be used as the field name, rather than the
+//! constant's name. For example:
+//! ```
+//! # use tracing::{span, Level};
+//! # fn main() {
+//! const RESOURCE_NAME: &str = "foo";
+//! // this span will have the field `foo = "some_id"`
+//! span!(Level::TRACE, "get", { RESOURCE_NAME } = "some_id");
+//! # }
+//!```
+//!
 //! The `?` sigil is shorthand that specifies a field should be recorded using
 //! its [`fmt::Debug`] implementation:
 //! ```