runtime/metrics: add Read examples

This change adds two examples of using the Read function: one that reads
one metric and one that reads all metrics.

Change-Id: I4940a44c9b1d65f3f7a1554e3145ff07e6492fc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/275855


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>