Fix noisy GRPC metadata errors.

[jgreat]

Unfortunately GRPC_VERBOSITY doesn't have a "none" value 😢

GRPC_VERBOSITY is used to set the minimum level of log messages printed by gRPC (supported values are DEBUG, INFO and ERROR). If this environment variable is unset, only ERROR logs will be printed.

How do we figure out what's causing this noisy error in the logs and fix the metadata or let the grpc server accept key=content-type value=text as valid.

E0324 19:08:30.384842702      34 hpack_parser.cc:1216]       Error parsing metadata: error=invalid value key=content-type value=text