tracing: allow setting event names in macros (#2699)

## Motivation

The motivation is #1426.  Currently, event names are set to a default
value of `event file:line`, which (1) doesn't allow for customization,
and (2) prevents events from working for some opentelemetry endpoints
(they often use the event name `exception` to designate something like a
panic).

## Solution

Went through the event macros, and added new parameterization that
allows for setting the `name`.  In addition, added some comments, and
reordering, to make life a bit better for the next person that comes
along to edit those macros.  Finally, added tests for the macro
expansion alongside the existing tests with a reasonable amount of
coverage (though, admittedly, more could be added for all of the macro
invocation types

Fixes #1426

tracing/src/lib.rs

@@ -214,7 +214,7 @@
 //! ### Configuring Attributes
 //!
 //! Both macros require a [`Level`] specifying the verbosity of the span or
-//! event. Optionally, the [target] and [parent span] may be overridden. If the
+//! event. Optionally, the, [target] and [parent span] may be overridden. If the
 //! target and parent span are not overridden, they will default to the
 //! module path where the macro was invoked and the current span (as determined
 //! by the subscriber), respectively.
@@ -237,7 +237,16 @@
 //! ```
 //!
 //! The span macros also take a string literal after the level, to set the name
-//! of the span.
+//! of the span (as above).  In the case of the event macros, the name of the event can
+//! be overridden (the default is `event file:line`) using the `name:` specifier.
+//!
+//! ```
+//! # use tracing::{span, event, Level};
+//! # fn main() {
+//! span!(Level::TRACE, "my span");
+//! event!(name: "some_info", Level::INFO, "something has happened!");
+//! # }
+//! ```
 //!
 //! ### Recording Fields
 //!