[7.6] Switch to AtomicLong for "IngestCurrent" metric to prevent negative values

[danhermann]

For monotonically-increasing metrics, the tradeoff of LongAdder's lower contention for non-atomic value reads tends not to cause problems. The "IngestCurrent" metric is more of a flag-type metric that should hover between zero and a small positive number with frequent increment and decrement operations. In that case, LongAdder's non-atomic reads can result in negative values being returned due to out-of-order application of the increment and decrement operations. Those negative values both confuse users and fail during serialization. For that specific metric, AtomicLong's guarantees are necessary.

Backport of #52581

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Stats)