* fncache: Pass non-cancellable context to `OnExpiry` callbacks
Call `OnExpiry` when `get()` replaces an expired entry on reload.
Previously, entries that expired between cleanup intervals were
silently dropped when a new request triggered a reload, skipping
the `OnExpiry` callback entirely.
Use `context.WithoutCancel(c.cfg.Context)` at both the
`removeExpiredLocked` and `get` call sites so that `OnExpiry` work
completes even after shutdown begins. `Shutdown` cancels
`c.cfg.Context` via `c.cancel()` before processing entries, but
these two call sites passed `c.cfg.Context` directly, meaning
goroutines spawned by `OnExpiry` could run with an
already-cancelled context.
Update the `OnExpiry` doc comment to warn that the cache mutex may
be held when the callback is invoked.
* app: Add active sessions Prometheus gauge
Add a `teleport_app_active_sessions` gauge labeled by app name that
tracks HTTP app sessions on each agent. The gauge increments when a
session chunk is created and decrements after the session chunk
finishes closing, so it reflects sessions still holding resources
(audit streams, disk I/O) rather than just sessions accepting new
requests.
TCP and MCP sessions are excluded because they bypass the session
chunk cache.
* fncache: Fix `OnExpires` typo in `Shutdown` doc comment
Correct the stale field name `OnExpires` to `OnExpiry` in the
`Shutdown` method's doc comment to match the actual field name in
`FnCacheConfig`.
Julia Ogris
6c8ea32bc2