serverStatus.concurrentTransactions.<read/write> has new metrics not gated by a feature flag. Please note these fields were added in 6.1.0-rc0

"out": total number of tickets checked out at this time
"available": total number of tickets available at this time
"totalTickets": total number of tickets allotted for the system
"addedToQueue": total number of operations waiting for a ticket over the lifetime of the ticketing system
"removedFromQueue": the total number of operations queued
"queueLength": the number of operations queued
"startedProcessing": the cumulative number of tickets that have begun processing (regardless of whether they were queued or retrieved a ticket immediately upon request)
"processing": number of tickets that are in use (calculated as startedProcessing - finishedProcessing)
"finishedProcessing": cumulative number of tickets that have finished processing (regardless of whether they were queued or retrieved a ticket immediately upon request)
"totalTimeProcessingMicros": cumulative time tickets have spent checked out
"canceled": the total number of ticket requests canceled
"newAdmissions": the total number of 'newAdmissions' to the system. A new admission denotes the first time a process admits for a ticket - the number excludes admissions that are restored after yield

Description of Linked Ticket

Right now the ticketholders expose the metrics dependent on the implementation. There are some common metrics that would be of interest across all ticketholders regardless of implementation.