tracing: add index API for `Field` (#2820)
## Motivation
Expose the index of the field in order to support cases such as sending field information
by index instead of by the string name in Tokio Console. Details:
https://github.com/tokio-rs/console/issues/462#issuecomment-1830842319
## Solution
Adds a new API which exposes the index of a field, which is how it is stored internally
(together with a reference to the `FieldSet` containing the ordered field names.
Signed-off-by: hi-rustin <rustin.liu@gmail.com>
Co-authored-by: Hayden Stainsby <hds@caffeineconcepts.com>
