roachtest: add cluster health metric to the end of tpcc run

[aliher1911]

Currently we don't check any cluster metrics post tpcc runs because
they don't affect functional correctness. To use perf runs as one of
quality signals we want to check if intents build up when running test.
To do this, we now collect number of intents as a separate report
json file and push it to cloud storage that holds stats.json.

Release note: None

[cockroach-teamcity]

This change is