Part of https://github.com/open-telemetry/opentelemetry-go/issues/7800
This adds Enabled checks to the generated semconv metric instruments. I
also opted to regenerate the 1.40.0 conventions given this is not a
behavioral change.
While implementing it, I did get the sense that this _should_ be
redundant. Callers should be checking Enabled before making these calls
because they should avoid building attribute sets. But it does still
make sense to have this as a fallback for cases where callers haven't
done that.
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Co-authored-by: Robert Pająk <pellared@hotmail.com>