Fix host metrics timeout and other improvements

Fixed

• Continue reporting non-disk host metrics when a mount is frozen.

• Fix host-metrics leaking zombie [timeout] processes in Alpine linux containers.

  Before this release AppSignal agent relied on a proper init process that reaps child processes killed by system timeout. Now the agent terminates and reaps unresponsive child processes in host-metrics collection and a subreaper is no longer required.

• Ensure sendError ends its span even when the callback throws.

View the Node.js package v3.8.1 changelog for more information.