Thread pool and read/write latency statistics 

Increases in pending tasks on thread pool statistics can indicate when to add additional capacity.

Cassandra maintains distinct thread pools for different stages of execution. Each of the thread pools provide statistics on the number of tasks that are active, pending, and completed. Trends on these pools for increases in the pending tasks column indicate when to add additional capacity. After a baseline is established, configure alarms for any increases above normal in the pending tasks column. Use nodetool tpstats on the command line to view the thread pool details shown in the following table.
Thread Pool statistics reported by nodetool tpstats
Thread Pool Description
AntiEntropyStage Tasks related to repair
CacheCleanupExecutor Tasks related to cache maintenance (counter cache, row cache)
CompactionExecutor Tasks related to compaction
CounterMutationStage Tasks related to leading counter writes
GossipStage Tasks related to the gossip protocol
HintsDispatcher Tasks related to sending hints
InternalResponseStage Tasks related to miscellaneous internal task responses
MemtableFlushWriter Tasks related to flushing memtables
MemtablePostFlush Tasks related to maintenance after memtable flush completion
MemtableReclaimMemory Tasks related to reclaiming memtable memory
MigrationStage Tasks related to schema maintenance
MiscStage Tasks related to miscellaneous tasks, including snapshots and removing hosts
MutationStage Tasks related to writes
Native-Transport-Requests Tasks related to client requests from CQL
PendingRangeCalculator Tasks related to recalculating range ownership after bootstraps/decommissions
PerDiskMemtableFlushWriter_* Tasks related to flushing memtables to a given disk
ReadRepairStage Tasks related to performing read repairs
ReadStage Tasks related to reads
RequestResponseStage Tasks for callbacks from intra-node requests
Sampler Tasks related to sampling statistics
SecondaryIndexManagement Tasks related to secondary index maintenance
ValidationExecutor Tasks related to validation compactions
ViewMutationStage Tasks related to maintaining materialized views

Read/Write latency metrics 

Cassandra tracks latency (averages and totals) of read, write, and slicing operations at the server level through StorageProxyMBean.