[receiver/snmpreceiver] fix scalar RA # check

[kuiperda]

Description:
Fixed a bug introduced by #25174

There was check that was supposed to ensure that all resource attributes added to a metric were scalar, but it was really checking if the number of resource attributes on a metric was the same as the total number of possible scalar resource attributes defined in the config. This check has been updated to explicitly verify that all of the resource attributes on the metric are scalar.

Bug examples:

• A column metric indexed by a column attribute has two scalar resource attributes with different names but the same oid value. They are the only two scalar RAs defined in the config. Since the oids are added to a map whose length is checked, they are seen as one value and the comparison fails so many resources are created instead of one.
• A column metric indexed by a column attribute has only one scalar resource attribute when two possible attributes are defined in the config. The check fails and many resources are created instead of one.

Testing:

• Test that a metric with one scalar resource attribute when two are available still creates only one resource
• First bug does not feel necessary to unit test, it has been tested locally and is fixed.

Documentation:

• N/A

[djaglowski]

LGTM. Just one nit.