grpc_logging interceptor logs full request bodies as ASCII byte arrays, not strings

[cboggs]

A prime example of this is the behavior seen on a failed push to an ingester where the entire push body is logged as a frighteningly large byte array.

Reference #89 and https://github.com/weaveworks/cortex/pull/709 for a more thorough conversation on the details.

The above-referenced weaveworks/common PR avoids this issue by simply allowing a per-service configuration that truncates these logs, omitting the request body.

It might be better, though, to retain the request bodies and simply log them as the strings they are, rather than their ASCII code representation.