[remappers/hostmetrics] Add support for `system.uptime` metric

[andrzej-stencel]

Current situation

The upstream Host Metrics receiver added support for system.uptime metric in v0.113.0. The Elastic System integration also supports the uptime metric.

Problem

The problem is, the Elastic Infra Metrics processor currently does not support the metric, so the upstream metric is not translated to ECS.

Proposed solution

Add the mapping from OTel system.metric to ECS in remappers/hostmetrics so that the Elastic Infra Metrics processor translates the OTel metric to ECS, so that the metric shows up in the System integration dashboards.

[rogercoll]

We are planning to remove the EIMP from the default configurations in EDOT 9.2, probably we should advise switching to the hostmetrics receiver + otel Elasticsearch exporter mode. At that time, we will move the EIMP to maintenance mode.

Also, the system.uptime metric will be enabled by default in EDOT's Otel default configurations, similar to the default system metricbeat metrics. https://github.com/elastic/opentelemetry-dev/issues/968