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

# Current situation

The upstream [Host Metrics receiver](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/v0.130.0/receiver/hostmetricsreceiver/README.md) added support for [system.uptime](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/v0.130.0/receiver/hostmetricsreceiver/internal/scraper/systemscraper/documentation.md#systemuptime) metric in [v0.113.0](https://github.com/open-telemetry/opentelemetry-collector-contrib/releases/v0.113.0). The Elastic [System integration](https://www.elastic.co/docs/reference/integrations/system#uptime) 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.