Rename sdk/metric/export package to sdk/metric/metricdata

[MrAlias]

Currently the sdk/metric/export package contains the data types that metric telemetry are represented by.

The name of this package is problematic. It is not uniquely descriptive of its contents and it conflicts with other packages we have previously published ^{1 2}.

Another proposal was to use the package name sdk/metric/data. This name still does not effectively represent the contents of the package as being related to metrics. It has a high probability of colliding with other data type packages we will add in the future.

I propose we rename it to sdk/metric/metricdata. This follows other "test" package names (e.g. http/httptest), but instead will refer to fact that package contains data types related to metrics.

Footnotes

1. https://pkg.go.dev/go.opentelemetry.io/otel/sdk/export ↩

2. https://pkg.go.dev/go.opentelemetry.io/otel/sdk/[email protected]/export ↩

[hanyuancheung]

done with #3014