opentelemetry: allocate fewer strings for recording events

[hawkw]

Motivation

Currently, the tracing-opentelemetry subscriber will allocate several
strings for opentelemetry key-value fields in its on_event
implementation. Some of these allocations are not necessary, since
opentelemetry::Value::String can take a Cow, allowing &'static strs
to be used without allocating a new String.

Since this happens for every event that's recorded to opentelemetry,
this probably has a meaningful performance impact.

Solution

This branch makes two primary changes:

• Change the on_event method to subscriber to use &'static strs for
  event targets when possible, similarly to how we did this for source
  locations in opentelemetry: forward event metadata #1911. This way, when events were not recorded via the
  tracing-log adapter, we will use the &'static tracing metadata
  string for their targets, rather than allocating a new String. New
  Strings are only allocated when an event came from the log crate
  and its target is not valid for the 'static lifetime.

• Use Level::as_str for the Level key-value field, instead of
  Level::to_string. to_string calls fmt::Display and returns a
  String, while as_str returns an &'static str. This way, levels
  will never allocate a String.

[jtescher]

🥳