tracing: reorganize benchmarks for comparability

[hawkw]

Currently, tracing's benchmark suite benchmarks the same behaviors
(e.g. creating a span, recording an event, etc) across a handful of
cases: with no default dispatcher, with a global default, and with a
scoped (thread-local) default. We use criterion's benchmark_group to
represent each kind of dispatcher, and bench_function for each
behavior being measured.

This is actually kind of backwards relative to how Criterion is
supposed to be used. bench_function is intended for comparing
different implementations of the same thing, with the
benchmark_group representing what's being compared. If we inverted the
structure of these benchmarks, Criterion would give us nicer plots that
would allow comparing the performance of each dispatch type on the same
task.

This PR reorganizes the benchmarks so that each behavior being tested
(such as entering a span or recording an event) is a benchmark_group,
and each dispatch type (none, global, or scoped) is a bench_function
within that group. Now, Criterion will generate plots for each group
comparing the performance of each dispatch type in that benchmark.

For example, we now get nice comparisons like this:

Unfortunately, this required splitting each benchmark type out into its
own file. This is because, once we set the global default dispatcher
within one benchmark group, it would remain set for the entire lifetime
of the process --- there would be no way to test something else with no
global default. But, I think this is fine, even though it means we now
have a bunch of tiny files: it also allows us to run an individual
benchmark against every combination of dispatch types, without having to
run unrelated benches. This is potentially useful if (for example)
someone is changing only the code for recording events, and not spans.