server: refactor sleep handling, allow access /metrics during sleep

[ngxson]

Overview

• server_routes will read cached response during sleep
• clarify the call order of on_sleeping_state

as a side-effect, /metrics can now be accessed during sleep

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: only for adding tests

[ServeurpersoCom]

I think /slots should keep resetting the idle timer, only /metrics should skip it, otherwise monitoring no longer keeps the server awake but still forces a model reload on every polling round.

• 3 nits :

1. cached_metrics keeps the buckets as they were, so a sleeping server reports its last throughput on prompt_tokens_seconds and predicted_tokens_seconds forever, a reset_bucket() on the cached copy would give the 0 that the window actually holds.

2. setting content_type before the is_error() check means an error result is now returned as JSON labelled text/plain, since error() only overwrites the status and the body.

3. tmp.n_idle_slots is never read, to_metrics() only emits n_processing_slots and n_tasks_deferred.

I pushed directly:

[ngxson]

for /slots vs /metrics, it's better to separate them to 2 different tasks, which was done in c1fc93f

that also resolved the n_idle_slots being unused

[ServeurpersoCom]

LGTM

[ServeurpersoCom]

Nice. Splitting the task type kills the root cause instead of patching the symptom, and keeping the last window until a scrape reads it is more correct than dropping it on sleep