Standardize log->metric builder and emitter functions in eng/common

See https://github.com/Azure/azure-sdk-for-cpp/pull/3357#discussion_r816243352

I think it will be good for us to provide standard metric object builders + writers that implement the base metric "spec" so the repos can't diverge and we don't end up with slightly different implementations. Things like common OSVmImage mappings, ISO8601 timestamp formats, etc.

Naive example:

```
$extraMetrics = @{ key1 = value1; key2 = value2 }
$metric = BuildMetric $extraMetrics # from eng/common
LogMetric $metric # from eng/common
```

@hallipr if this is a duplicate or already planned/done, feel free to close.