Send non-scalar metrics values using msgpack

[alahiff]

Scalar metrics are done like this already (hopefully - the original Python API did).

[kzscisoft]

@alahiff I believe this is already the case

[kzscisoft]

Update: This is indeed missing, will add the relevant function override to ensure this to GridMetrics class as in Metrics:

def _post_single(self, **kwargs) -> dict[str, typing.Any]:
      return super()._post_single(is_json=False, **kwargs