Separate the sdk-rs's Metric into TextMetric and NumericMetric

Currently there's a small issue with numeric values quietly being truncated, which can cause a confusion at times. Systems that are interested in avoiding rounding/truncation might want to display an exact textual representation of a metric (e.g. avoid rounding 0.0015 to 0.002). 

It would be nice to extend the status protocol to support TextMetric and Numeric Metric as separate types