MetricData messages in mdm topic poorly defined. make versioning explicit?

[Dieterbe]

see discussion at #2011 (comment)

MetricData is currently assumed, not explicitly defined via a format byte. In practice, the first byte will typically be 0x89, but as @shanson7 points out, there could be a bit of a wider range in play in practice for users who have their own custom publishers that don't encode full MetricData payloads. I'm not aware of anying doing that, all "official" tools use 0x89, and all official docs refer to encoding the full MetricData payload, so it seems like a fair position to assume anything but 0x89 is unsupported. OTOH, in practice, tolerating the wider range wouldn't really cost us anything.