## Problem
Currently to know how long pageserver startup took requires inspecting
logs.
## Summary of changes
`pageserver_startup_duration_ms` metric is added, with label `phase` for
different phases of startup.
These are broken down by phase, where the phases correspond to the
existing wait points in the code:
- Start of doing I/O
- When tenant load is done
- When initial size calculation is done
- When background jobs start
- Then "complete" when everything is done.
`pageserver_startup_is_loading` is a 0/1 gauge that indicates whether we are in the initial load of tenants.
`pageserver_tenant_activation_seconds` is a histogram of time in seconds taken to activate a tenant.
Co-authored-by: Joonas Koivunen <joonas@neon.tech>
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