[metrics] Add RequestMetricsReport

This function can be used to explicit request a metrics report in
circumstances where the normal policies do not fit as well.

Usage:

    Runtime::Current()->GetMetrics()->RequestMetricsReport()

Test: m com.android.art
Change-Id: Ie2a9aa1b5da7b073d6d06f1647d5b8d72439fcb5
4 files changed