feat(health): make health API storage-aware

This change makes the GET request of /health to ensure that all
connected storage shards accepts `ping` command before responding
200; otherwise, 503.

The HEAD request is unchanged.

Closes-Bug: 1243268
Change-Id: I75c3b46ccb1a5a66e6c12766d3e451fd3478822c
Merge "feat(health): make health API storage-aware"