Make SystemMetricsMonitor.Configuration .labels, and .dimensions public again

[mman]

This PR addresses issue #96 by allowing the SystemMetricsMonitor.Configuration to be initialized with default labels and custom dimensions to attach prometheus dimensions like pod to every collected metric like this:

let registry = PrometheusCollectorRegistry()
let factory = await PrometheusMetricsFactory(registry: registry)
MetricsSystem.bootstrap(factory)
let monitor = SystemMetricsMonitor(configuration:
    .init(labels: .init(), dimensions: [("pod", hostname)]), metricsFactory: factory, logger: logger)
)