Inter-event timing metrics on top of [tracing].

This crate provides a tracing::Subscriber that keeps statistics on inter-event timing information. More concretely, given code like this:

rust use tracing::*; use tracing_timing::{Builder, Histogram}; let subscriber = Builder::from(|| Histogram::new_with_max(1_000_000, 2).unwrap()).build(); let dispatcher = Dispatch::new(subscriber); dispatcher::with_default(&dispatcher, || { trace_span!("request").in_scope(|| { // do a little bit of work trace!("fast"); // do a lot of work trace!("slow"); }) });

You can produce something like this:

```text fast: 50µs | 100µs | 150µs | * 200µs | **** 250µs | *** 300µs | 350µs | ** 400µs | * 450µs | 500µs |

slow: 550µs | 600µs | 650µs | * 700µs | ****** 750µs | * 800µs | * 850µs | * 900µs | ```

When TimingSubscriber is used as the tracing::Dispatch, the time between each event in a span is measured using [quanta], and is recorded in "[high dynamic range histograms]" using [hdrhistogram]'s multi-threaded recording facilities. The recorded timing information is grouped using the SpanGroup and EventGroup traits, allowing you to combine recorded statistics across spans and events.