### What changes were proposed in this pull request?
`ExternalBlockHandler` exposes 4 metrics which are Dropwizard `Timer` metrics, and are named with a `millis` suffix:
```
private final Timer openBlockRequestLatencyMillis = new Timer();
private final Timer registerExecutorRequestLatencyMillis = new Timer();
private final Timer fetchMergedBlocksMetaLatencyMillis = new Timer();
private final Timer finalizeShuffleMergeLatencyMillis = new Timer();
```
However these Dropwizard Timers by default use nanoseconds ([documentation](https://metrics.dropwizard.io/3.2.3/getting-started.html#timers)).
This causes `YarnShuffleServiceMetrics` to expose confusingly-named metrics like `openBlockRequestLatencyMillis_nanos_max` (the actual values are currently in nanos).
This PR adds a new `Timer` subclass, `TimerWithCustomTimeUnit`, which accepts a `TimeUnit` at creation time and exposes timing information using this time unit when values are read. Internally, values are still stored with nanosecond-level precision. The `Timer` metrics within `ExternalBlockHandler` are updated to use the new class with milliseconds as the unit. The logic to include the `nanos` suffix in the metric name within `YarnShuffleServiceMetrics` has also been removed, with the assumption that the metric name itself includes the units.
### Does this PR introduce _any_ user-facing change?
Yes, there are two changes.
First, the names for metrics exposed by `ExternalBlockHandler` via `YarnShuffleServiceMetrics` such as `openBlockRequestLatencyMillis_nanos_max` and `openBlockRequestLatencyMillis_nanos_50thPercentile` have been changed to remove the `_nanos` suffix. This would be considered a breaking change, but these names were only exposed as part of #32388, which has not yet been released (slated for 3.2.0). New names are like `openBlockRequestLatencyMillis_max` and `openBlockRequestLatencyMillis_50thPercentile`
Second, the values of the metrics themselves have changed, to expose milliseconds instead of nanoseconds. Note that this does not affect metrics such as `openBlockRequestLatencyMillis_count` or `openBlockRequestLatencyMillis_rate1`, only the `Snapshot`-related metrics (`max`, `median`, percentiles, etc.). For the YARN case, these metrics were also introduced by #32388, and thus also have not yet been released. It was possible for the nanosecond values to be consumed by some other metrics reporter reading the Dropwizard metrics directly, but I'm not aware of any such usages.
### How was this patch tested?
Unit tests have been updated.
Closes#33116 from xkrogen/xkrogen-SPARK-35259-ess-fix-metric-unit-prefix.
Authored-by: Erik Krogen <xkrogen@apache.org>
Signed-off-by: yi.wu <yi.wu@databricks.com>
(cherry picked from commit 70a15868fc)
Signed-off-by: yi.wu <yi.wu@databricks.com>
23e48dbf77