KAFKA-20796: Fix num-open-iterators metric for timestamped range queries

[dino2895]

MeteredTimestampedKeyValueStoreIterator registered itself in
openIterators, but did not update numOpenIterators. As a result,
iterators created by IQv2 RangeQuery and TimestampedRangeQuery
were not included in the num-open-iterators metric.

This change increments the counter when the iterator is created and
decrements it when the iterator is closed.

Regression tests verify that the metric transitions from 0 to 1 while
the iterator is open and returns to 0 after it is closed for both query
types.

Testing:

• ./gradlew :streams:spotlessJavaCheck
• ./gradlew :streams:checkstyleMain :streams:checkstyleTest
• ./gradlew :streams:test