[Metrics UI] Remove Snapshot API from Inventory View and use Metrics API

[Zacqary]

The new Metrics API was built to support queries from both the Inventory View and the Metrics Explorer, but Inventory is still querying the Snapshot API.

We should switch Inventory over to the new API.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)