runtime: de-duplicate span scavenging
runtime: de-duplicate span scavenging

Currently, span scavenging was done nearly identically in two different
locations. This change deduplicates that into one shared routine.

For #14045.

Change-Id: I15006b2c9af0e70b7a9eae9abb4168d3adca3860
Reviewed-on: https://go-review.googlesource.com/c/139297
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

akutz/go,josharian/go,cloudflare/go,cloudflare/go,josharian/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,josharian/go,akutz/go,akutz/go,josharian/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,josharian/go,CAFxX/go,akutz/go,CAFxX/go,akutz/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,josharian/go,CAFxX/go,josharian/go,josharian/go,CAFxX/go,akutz/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,akutz/go,nawawi/go,golang/go