Refactor out autocollection of metrics

Since system metrics are auto-collected in the [OpenTelemetry](https://github.com/open-telemetry/opentelemetry-python/pull/652) repo, refactor out the system metrics in [autocollection](https://github.com/microsoft/opentelemetry-azure-monitor-python/tree/master/azure_monitor/src/azure_monitor/sdk/auto_collection) to leverage this.