[Bug]: `IllegalStateChangeError` in `delete_metrics` task

[un-def]

Steps to reproduce

Start the server and wait

Actual behaviour

No response

Expected behaviour

No response

dstack version

0.19.9

Server logs

InvalidRequestError: This session is provisioning a new connection; concurrent operations are not permitted (Background on this error at: https://sqlalche.me/e/20/isce)
  File "dstack/_internal/server/db.py", line 87, in get_session
    yield session
  File "dstack/_internal/server/background/tasks/process_metrics.py", line 53, in delete_metrics
    await asyncio.gather(
  File "sqlalchemy/ext/asyncio/session.py", line 463, in execute
    result = await greenlet_spawn(
  File "sqlalchemy/util/_concurrency_py3k.py", line 190, in greenlet_spawn
    result = context.switch(*args, **kwargs)
  File "sqlalchemy/orm/session.py", line 2365, in execute
    return self._execute_internal(
  File "sqlalchemy/orm/session.py", line 2241, in _execute_internal
    conn = self._connection_for_bind(bind)
  File "sqlalchemy/orm/session.py", line 2110, in _connection_for_bind
    return trans._connection_for_bind(engine, execution_options)
  File "<string>", line 2, in _connection_for_bind
    # Copyright (C) 2005-2025 the SQLAlchemy authors and contributors
  File "sqlalchemy/orm/state_changes.py", line 103, in _go
    self._raise_for_prerequisite_state(fn.__name__, current_state)
  File "sqlalchemy/orm/session.py", line 990, in _raise_for_prerequisite_state
    raise sa_exc.InvalidRequestError(

IllegalStateChangeError: Method 'close()' can't be called here; method '_connection_for_bind()' is already in progress and this would cause an unexpected state change to <SessionTransactionState.CLOSED: 5> (Background on this error at: https://sqlalche.me/e/20/isce)
  File "apscheduler/executors/base.py", line 181, in run_coroutine_job
    retval = await job.func(*job.args, **job.kwargs)
  File "dstack/_internal/server/background/tasks/process_metrics.py", line 52, in delete_metrics
    async with get_session_ctx() as session:
  File "contextlib.py", line 231, in __aexit__
    await self.gen.athrow(typ, value, traceback)
  File "dstack/_internal/server/db.py", line 86, in get_session
    async with _db.get_session() as session:
  File "sqlalchemy/ext/asyncio/session.py", line 1031, in close
    await greenlet_spawn(self.sync_session.close)
  File "sqlalchemy/util/_concurrency_py3k.py", line 190, in greenlet_spawn
    result = context.switch(*args, **kwargs)
  File "sqlalchemy/orm/session.py", line 2521, in close
    self._close_impl(invalidate=False)
  File "sqlalchemy/orm/session.py", line 2590, in _close_impl
    transaction.close(invalidate)
  File "<string>", line 2, in close
    # Copyright (C) 2005-2025 the SQLAlchemy authors and contributors
  File "sqlalchemy/orm/state_changes.py", line 121, in _go
    raise sa_exc.IllegalStateChangeError(

Additional information

No response