ExpandedPostings: Allow promise execution when context was canceled but other callers are still waiting for results

[colega]

Is your feature request related to a problem? Please describe.
See #330 (comment)

When the context of the promise creator for ExpandedPostings is canceled, the promise will be canceled, even if there are other callers waiting for the promise result.

Describe the solution you'd like
Promise should be still executed when someone is waiting for its results.

Describe alternatives you've considered
Just let it be as it's now, because probably the entire request was canceled (promise share requests from same sharded query, if one sharded query is canceled, the other one is too, isn't it true?)

Additional context
#329

[colega]

@pracucci I think I'll close this issue, this doesn't seem to happen, and the day it will happen it will probably be because the entire sharded request was cancelled, as that is where we hit the promise.

[aknuds1]

Similar to grafana/mimir-prometheus#548.

[dimitarvdimitrov]

This is still relevant. It happened today:

Context tracking is already implemented for prometheus in grafana/mimir-prometheus#644 we can probably just copy that implementation