Prometheus exporter: omit empty otel_scope_info and otel_target_info metrics

[dashpole]

Description

otel_scope_info and target_info should now only be generated when scope attributes and resource attributes are non-empty.

From https://github.com/open-telemetry/opentelemetry-specification/pull/3660:

Prometheus exporters SHOULD generate an Info-typed metric named otel_scope_info for each Instrumentation Scope with non-empty scope attributes.

In SDK Prometheus (pull) exporters, resource attributes SHOULD be converted to a single target_info metric if the resource is not empty;