grpc binlog should support headers only or messages only

Right now specifying headers-only would make us log messages truncated to 0 bytes, and specifying messages-only would make us log headers truncated to 0 bytes. This is not compliant with the grfc.

Start by locating the Java gRPC binary logging implementation and its existing tests for header and message truncation. Reproduce the headers-only and messages-only cases, then verify that each mode records only its selected content without truncating the other field to zero bytes.

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