runtime/metrics: add a note about floating-point values to package docs

This change adds a note to the package documentation that the package
will never produce a NaN or infinity, to help ease usability.

Change-Id: I72ff6ab636ca23722a68ef11e707c68b0724ac04
Reviewed-on: https://go-review.googlesource.com/c/go/+/275854


Run-TryBot: Michael Knyszek <mknyszek@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>