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 open-telemetry/opentelemetry-specification#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;