GMP exporter should convert Int values to Double

[dashpole]

GMP only accepts Double values. Sending it an int will cause the metric to be rejected.
This is not correct. GMP accepts int and double values.

This can be done with the transform processor today, although I think this needs to be filtered so it only applies to int valued datapoints:

      transform/inttodouble:
        metric_statements:
          - context: datapoint
            statements:
              # GMP metrics are expected to be double
              - set(value_double, Double(value_int))

Ideally, the exporter should handle this, rather than requiring users to add processors. I encountered this when using the spanmetrics exporter, which produces int-valued metrics.

[dashpole]

This is tricky. If we do this in the exporter, existing users will encounter errors when they upgrade, since their existing metric descriptors are the wrong type. We would likely need to make this opt-in behavior.

[aabmass]

If GMP only supports doubles, how could their descriptors have the wrong type on upgrade?

[dashpole]

I think I must have been mistaken. There was a user that was able to write int metrics, and had problems switching to double.

[dashpole]

Confirmed that GMP accepts both int and double metrics

[dashpole]

In that case, we should probably not change the value type. If users have the same metric in both INT and Double, they can use the above workaround to merge them together.

[dashpole]

Actually, we should document this behavior on the exporter