[doc][monitoring][txn] add doc for transaction metrics (#15218)
[doc][monitoring][txn] add doc for transaction metrics (#15218)


massakam/pulsar,massakam/pulsar,yahoo/pulsar,massakam/pulsar,merlimat/pulsar,massakam/pulsar,yahoo/pulsar,massakam/pulsar,yahoo/pulsar,merlimat/pulsar,massakam/pulsar,yahoo/pulsar,yahoo/pulsar,merlimat/pulsar,yahoo/pulsar,merlimat/pulsar,merlimat/pulsar,merlimat/pulsar