Metrics: Improve handling of complex numbers

[duncanpo]

Adding or recording complex values currently returns an obscure error message:

>> record(h, 2+3i)
Error using gateway
Can't convert this element

Error in  .  (line 55)
                [varargout{1:nargout}] = libmexclass.proxy.gateway("MethodCall", obj.ID, methodName, methodArgs);

Error in opentelemetry.metrics.Histogram/record (line 35)
                    obj.Proxy.record(value);

1. We should not error. Invalid inputs should be a no-op.
2. Consider a warning

[duncanpo]

In Metrics branch