## What changes were proposed in this pull request?

This patch adds Coda Hale metrics for instrumenting the `LiveListenerBus` in order to track the number of events received, dropped, and processed. In addition, it adds per-SparkListener-subclass timers to track message processing time. This is useful for identifying when slow third-party SparkListeners cause performance bottlenecks.

See the new `LiveListenerBusMetrics` for a complete description of the new metrics.

## How was this patch tested?

New tests in SparkListenerSuite, including a test to ensure proper counting of dropped listener events.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #18083 from JoshRosen/listener-bus-metrics.