Prometheus exporter: emit target_info metric using Resource

[dashpole]

From the spec for handling resource in prometheus exporters:

In SDK Prometheus (pull) exporters, resource attributes SHOULD be converted to a single target_info metric; otherwise, they MUST be dropped, and MUST NOT be attached as labels to other metric families. The target_info metric MUST be an info-typed metric whose labels MUST include the resource attributes, and MUST NOT include any other labels. There MUST be at most one target_info metric exposed on an SDK Prometheus endpoint.

The Prometheus exporter currently doesn't use resource attributes for anything.