Consistent mc NaN output for "empty" metric values + middleware blob output option

[planetchili]

Review all metrics (particularly frame metrics) and identify which can yield empty values and under what conditions. mc system should output NaN or other nullish signal for these consistently. eg. msGPULatency, msGPUBusy, etc.

API should expose opt in setting to output NaN for all nullish values, instead of the current 0 output.