debug-interface: client improvements
debug-interface: client improvements

Some minor improvements to the debug-interface client:

* Enable creation from a Url
* Check for HTTP success before trying to deserialize the response when
  querying metrics

libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra