runtime/metrics: add a note about floating-point values to package docs
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 <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>

golang/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go