Update host-metrics-receiver.rst

the latest release included the system scraper in the host metrics receiver which outputs the metric system.uptime (maybe more). 
please consider changing the documentation on both default configuration files and the receiver documentation.

Author: mabac16

gdi/opentelemetry/components/host-metrics-receiver.rst

@@ -11,6 +11,7 @@ The host metrics receiver generates metrics scraped from host systems when the C
 
 By default, the host metrics receiver is activated in the Splunk Distribution of OpenTelemetry Collector and collects the following metrics:
 
+- System metrics
 - CPU usage metrics
 - Disk I/O metrics
 - CPU load metrics
@@ -91,6 +92,10 @@ Scrapers extract data from endpoints and then send that data to a specified targ
       - Description
    - 
 
+      - ``system``
+      - System metrics
+   - 
+
       - ``cpu``
       - CPU utilization metrics
    -