Add documentation for newly added metrics

[kjoseph07]

The metrics documentation website page needs to be updated for all newly added metrics: https://opensearch.org/docs/latest/monitoring-plugins/pa/reference/
This can be done by modifying https://github.com/opensearch-project/documentation-website/blob/main/_monitoring-plugins/pa/reference.md

The following metrics need to be added to the documentation website:

@dhwanilpatel
#466

• Data_RetryingPendingTasksCount
• Master_ThrottledPendingTasksCount

@amathur1893
#436

• Shard_State

#470

• LeaderCheck_Latency
• FollowerCheck_Failure
• LeaderCheck_Failure
• FollowerCheck_Latency

#563

• ClusterApplierService_Latency
• ClusterApplierService_Failure

#584

• PublishClusterState_Latency
• PublishClusterState_Failure

@psychbot
#560

• Indexing_Pressure_Current_Limits
• Indexing_Pressure_Current_Bytes
• Indexing_Pressure_Last_Successful_Timestamp
• Indexing_Pressure_Rejection_Count
• Indexing_Pressure_Average_Window_Throughput

@mitalawachat
#571

• AdmissionControl_ThresholdValue
• AdmissionControl_RejectionCount
• AdmissionControl_CurrentValue

@meetshah777
#583

• Election_Term

@eirsep
#70

• Thread_Waited_Event
• Thread_Waited_Time

[kkhatua]

Reached out to individual contributors for providing documentation update.