Add hooks/apis to support writing of coverage counter data and
meta-data under user control (from within an executing "-cover"
binary), so as to provide a way to obtain coverage data from programs
that do not terminate. This patch also adds a hook for clearing the
coverage counter data for a running program, something that can be
helpful when the intent is to capture coverage info from a specific
window of program execution.
Updates #51430.
Change-Id: I34ee6cee52e5597fa3698b8b04f1b34a2a2a418f
Reviewed-on: https://go-review.googlesource.com/c/go/+/401236
Reviewed-by: David Chase <drchase@google.com>
