Consider to remove IntExemplar and offer only DoubleExemplar (a.k.a Exemplar)

[bogdandrutu]

This decision was made in the initial beta release of the metrics, but after using this data model in the collector for some time, I came to the conclusion that they are a bit harmful (I have a bunch of places where I have to deal with conversion for this extra type). By using only one Exemplar we may lose precision for the value, but how large should the value be to have this problem, based on the IEEE 754 the maximum value is ~2^53 (I found on a stackoverflow thread that the value seem to be 9007199254740992) to not lose precision at all. I think this is a reasonable compromise for usability.

The deprecation will work this way:

• Rename current DoubleExemplar to just Exemplar.
• Everywhere where we use IntExemplar deprecate that field and replace it with Exemplars.
• When receiving a deprecated IntExemplar field, receiver can convert it into the new Exemplar field, only operation needed is to convert the int64 value into double.
• In the collector we will do this in the OTLP receiver, and will remove access to the deprecated field.

/cc @jmacd @open-telemetry/specs-metrics-approvers

[jmacd]

Strong support, like #257.