A list of metadata pairs, used in the payload of client header,
server header, and server trailer.
Implementations may omit some entries to honor the header limits
of GRPC_BINARY_LOG_CONFIG.

Header keys added by gRPC are omitted. To be more specific,
implementations will not log the following entries, and this is
not to be treated as a truncation:
- entries handled by grpc that are not user visible, such as those
  that begin with 'grpc-' (with exception of grpc-trace-bin)
  or keys like 'lb-token'
- transport specific entries, including but not limited to:
  ':path', ':authority', 'content-encoding', 'user-agent', 'te', etc
- entries added for call credentials

Implementations must always log grpc-trace-bin if it is present.
Practically speaking it will only be visible on server side because
grpc-trace-bin is managed by low level client side mechanisms
inaccessible from the application level. On server side, the
header is just a normal metadata key.
The pair will not count towards the size limit.

Protobuf type grpc.binarylog.v1.Metadata