s3 backend logs "unsupported value type"

[joe-elliott]

Describe the bug
In a number of places the s3 backend logs "unsupported value type" b/c it's being passed a struct. For example:

https://github.com/grafana/tempo/blob/master/tempodb/backend/s3/s3.go#L191

Review the code and pass an appropriate value here. Perhaps size.Size? Also note that this happens in multiple places so please try to find and correct each one.

To Reproduce
Steps to reproduce the behavior:

1. Use the S3 backend.

Expected behavior
Actual size in bytes to be logged.

Environment:
N/A

Additional Context