Add client_requests timing module to metrics

[michaeldjeffrey]

Questions:

• Is there a way we can handle repos that take this as a dependency having a different metrics crate version than oracles?
  • Ended up updating metrics to latest api style and putting a >=0.22 on the dep.

---

• Add a timing span to anything that can be instrumented and returns a Result.

  Example: ```ignore let client = GatewayClient::new(channel);

  client.info(req) .with_timing("iot_fetch_info") .await?; ```

  This will result in a prometheus metric

  client_request_duration_seconds{name = "iot_fetch_info", quantile="xxx"}

• Install the ApiTimingLayer.

  Adding .with_span_events(FmtSpan::CLOSE) to a regular format layer will print the timing spans to stdout as well.

  Example: ignore tracing_subscriber::registry() .with(tracing_subscriber::fmt::layer().with_span_events(FmtSpan::CLOSE)) .with(metrics::client_requests::client_request_timing_layer("histogram_name")) .init();

• Remove unused install_metrics function, replace with nested install function that start_metrics delegates to. This allows us to start metrics in tests without needing to make a Settings struct.