update promMetrics interface and use collect fn

[busma13]

This PR makes the following changes:

• adds a static object counters to CountByField to track all field counts across multiple operations
• add default labels from worker to count_by_field_count_total metric
• addCounter now uses a collect() function to increment count only at time of /metrics endpoint scrape
• _incMetric now increments its value in counters instead of the promMetrics directly
• tests that use promMetrics now manually init the API
• tests use test.context.apis.scrapePromMetrics() to get metrics directly from prom-client