Metric Reporting Frequency

The agent currently has [a single configuration field](https://www.elastic.co/guide/en/apm/agent/nodejs/current/configuration.html#metrics-interval) (`metricsInterval`) that controls how often metrics are reported.

We should consider whether we want _all_ metrics to continue obeying this global setting, or if it's worth having more fine grained control over which metrics can be reported more (or less) often. Gauge metrics, in particular, can benefit from more frequent reporting. A "bursty" issue that 1. falls in-between reporting frequencies and 2. is measured by a Gauge might be missed when the reporting period is too long.