Return the gRPC metadata to attach to the next outgoing RPC. Keys are interpreted as header names (gRPC will lowercase them); values are arbitrary strings.
May return a plain object or a Promise for async refresh. If this
method throws or the returned Promise rejects, the affected call
is cancelled with gRPC status UNAUTHENTICATED.
Pluggable authentication for Oxia client RPCs. Implementations return a map of gRPC metadata entries to attach to every outgoing call (for example,
{ authorization: 'Bearer <token>' }). Authentication.generateCredentials is invoked once per RPC, so dynamic token-refresh schemes work by returning a fresh value on each call.