Default returns a metric Middleware under the namespace "nirvana" for restful descriptor.

Descriptor returns a descriptor for the API; it must be configured to a server in order to serve the metric API.

Restful returns a metrics Middleware for Restful Descriptors built from the given options.

RPC returns a metrics Middleware for RCP Descriptors built from the given options.

RPCDescriptor returns a rpc descriptor for the API; it must be configured to a server in order to serve the metric API.