Invalid instance bootup time is collected by the prometheus

[Gary.Wang]

When investigating ams log shared by a customer, we discovered the following error

```
2024-05-21T08:46:02Z ams.ams[1178965]: I0521 08:46:02.067671 1178965 nodes.go:301] Done configuring remote LXD instance
2024-05-21T09:18:24Z ams.ams[1178965]: W0521 09:18:24.923495 1178965 collector.go:411] Collector: Instance cp66di23ho6aji4v4b7g has an invalid BootFinishedAt
2024-05-21T10:06:40Z ams.ams[1178965]: W0521 10:06:40.901117 1178965 collector.go:411] Collector: Instance cp673ja3ho6aji4v4bc0 has an invalid BootFinishedAt
2024-05-21T10:08:55Z ams.ams[1178965]: W0521 10:08:55.989732 1178965 collector.go:411] Collector: Instance cp675023ho6aji4v4bd0 has an invalid BootFinishedAt
2024-05-21T11:28:24Z ams.ams[1178965]: W0521 11:28:24.065135 1178965 collector.go:411] Collector: Instance cp68aga3ho6aji4v4c20 has an invalid BootFinishedAt
2024-05-21T11:41:23Z ams.ams[1178965]: W0521 11:41:23.782308 1178965 collector.go:411] Collector: Instance cp68gj3hmpmkt4var9d0 has an invalid BootFinishedAt
2024-05-29T02:11:24Z ams.ams[1178965]: W0529 02:11:24.562805 1178965 collector.go:411] Collector: Instance cpb8tdbhmpmkt4vardg0 has an invalid BootFinishedAt
2024-05-29T02:29:29Z ams.ams[1178965]: W0529 02:29:29.025321 1178965 collector.go:411] Collector: Instance cpb95ra3ho6aji4v4f00 has an invalid BootFinishedAt

```
As a consequence, the bootup time collected by the metrics always remain 0

This implies a race condition between the time an instance's bootup is recorded and the instance status changing to api.TaskStatusRunning.