### What changes were proposed in this pull request?
Ignore internal accumulators that use unrecognized types rather than crashing so that an event log containing such accumulators can still be converted to JSON and logged.
### Why are the changes needed?
A user may use internal accumulators by adding the `internal.metrics.` prefix to the accumulator name to hide sensitive information from UI (Accumulators except internal ones will be shown in Spark UI).
However, `org.apache.spark.util.JsonProtocol.accumValueToJson` assumes an internal accumulator has only 3 possible types: `int`, `long`, and `java.util.List[(BlockId, BlockStatus)]`. When an internal accumulator uses an unexpected type, it will crash.
An event log that contains such accumulator will be dropped because it cannot be converted to JSON, and it will cause weird UI issue when rendering in Spark History Server. For example, if `SparkListenerTaskEnd` is dropped because of this issue, the user will see the task is still running even if it was finished.
It's better to make `accumValueToJson` more robust because it's up to the user to pick up the accumulator name.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
The new unit tests.
Closes#28744 from zsxwing/fix-internal-accum.
Authored-by: Shixiong Zhu <zsxwing@gmail.com>
Signed-off-by: Shixiong Zhu <zsxwing@gmail.com>
b333ed0c4a