[server] Monitor and optimize Workspace Start

[svenefftinge]

This ticket is about running an optimization cycle (harvesting the top 3 performance improvements) for the workspace startup times between user sends request with context URL and ws-manager receives workspace spec.

Ideally, we introduce some observability metrics for this as well, so we can continuously monitor this.
Could be even a great long-standing success-criteria.

[geropl]

@AlexTugarev Moved to "In progess" based on your assignment 😉