testing: add an example showcasing B.RunParallel with B.ReportMetric

This commit was dedicated to adding an example of using B.ReportMetrics
with B.RunParallel called ExampleB_ReportMetric_parallel. In this
example, the same algorithm for ExampleB_ReportMetric was used, instead
with a concurrent for loop using PB.Next instead of a standard one.
There is also notes noting when to use the B.ReportMetric methods when
running concurrent testing.

Fixes #50756
Change-Id: I2a621b4e367af5f4ec47d38a0da1035a8d52f628
Reviewed-on: https://go-review.googlesource.com/c/go/+/437815
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

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