Schema transformations should be able to convert metric units

[pyohannes]

What are you trying to achieve?

Recently the default unit for duration metrics was changed from milliseconds to seconds. This resulted (and will result) in breaking changes to existing metrics semantic conventions. Some related PRs/issues:

• Change http.server.duration and http.client.duration units to seconds #3390
• BREAKING: Use seconds as default duration for FaaS duration histograms semantic-conventions#384
• Use seconds as default duration for RPC metrics semantic-conventions#383
• Use seconds as default duration for DB client metrics semantic-conventions#381

This change could be covered by automatic schema transformations by specifying the metric name, the old unit, the new unit, and a conversion factor.

Such a schema transformation would help telemetry consumers to mitigate the pain of such breaking changes, as data points from a metric from different versions of semantic conventions with different units could be aggregated into the same metric. This can help avoid severe disruptions for telemetry consumers.

Furthermore, the possibility of covering such changes by schema transformations gives more flexibility to semantic conventions authors, as without such a mechanism changing units for stable metrics would be impossible.

[tigrannajaryan]

I support this. Any transformation that we notice happens often in semconv and is possible to process in a stateless manner by just looking at an individual metric data point should be a candidate for becoming a supported transform in the Schema file.

[tigrannajaryan]

We need more community feedback and use cases to decide on the priority of this.