restore: add progres idx to logged span message

[dt]

This helps line it up with the pprof labels

Release note: none. Epic: none.

[blathers-crl[bot]]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[dt]

TFTR!

bors r+

[craig[bot]]

Build succeeded:

• Bazel Essential CI (Cockroach)