Minor improvements to GCS driver and Query Path

[annanay25]

What this PR does:
Bunch of small improvements:

• replace the use of ioutil.ReadAll() with ReadAllWithEstimate with a predefined buffer size, using the returned object attributes in tempodb/backend/gcs and tempodb/backend/s3
• add ContentLength header in span id deduper middleware
• update gcs driver version from v1.12.0 => v1.15.0. Primarily to catch the following change

// configureHTTP2 configures the ReadIdleTimeout HTTP/2 option for the
// transport. This allows broken idle connections to be pruned more quickly,
// preventing the client from attempting to re-use connections that will no
// longer work.

• fix a panic in query frontend by checking for nil response

Which issue(s) this PR fixes:
Fixes #na

Checklist

• Tests updated
• NA Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]