Add total successful task count in nodeStats API of SearchBackpressureService.

[kaushalmahi12]

Is your feature request related to a problem? Please describe.
Adding total successful task in searchShardTaskStats and searchTaskStats pojos to support autotune of heap based thresholds. Since the SBP Service node stats only include cancellation stats for shard and coordinator level requests, it is impossible to calculate the cancellation percentage due to a resource.

Describe the solution you'd like
We want to include total successful task count in the searchbackpressure stats at shard and coordinator level data.

Describe alternatives you've considered
N/A.

Additional context
We are planning to make use of this field to auto tune the SBP heap based cancellation thresholds at shard and coordinator level.

Related issues

• RCA Issue

[PritLadani]

Shall we also include a metric suggesting whether the node is under duress or not? It will be useful for investigation.
IsNodeUnderDuress can be added in the SearchBackpressureStats pojo.