gRPC per-signal methods
protobuf *ServiceClient
private const METHODS = [ Signals::TRACE => '/opentelemetry.proto.collector.trace.v1.TraceService/Export', Signals::METRICS => '/opentelemetry.proto.collector.metrics.v1.MetricsService/Export', Signals::LOGS => '/opentelemetry.proto.collector.logs.v1.LogsService/Export', ];