[exporter/signalfx] Use logical CPU count for num processors

[dmitryax]

Updates the default SignalFx translation rules to copy system.cpu.logical.count to cpu.num_processors instead of deriving the value by counting per-core cpu.idle series. The raw system.cpu.logical.count metric is default-excluded, so default output stays SignalFx-compatible while users can add system.cpu.logical.count to include_metrics if they want to publish both metrics.