Unreachable connections should be excluded from response time statistics

### Prerequisites

- [x] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions) and found no answer

- [x] I have searched other issues and found no duplicates

- [x] I want to request a feature or enhancement and not ask a question

### The problem

Currently, during network blackouts (which can occur due to ISP maintenance or regular reboots), the AGH includes the i/o timeout in the upstream response time calculation. This results in an extremely high response time value on the dashboard, which is misleading and has no bearing on the upstream connection.

### Proposed solution

I propose adding a separate section to indicate this condition instead of displaying an inaccurate value.

### Alternatives considered and additional information

I’m not certain if load-balancing mode uses this statistic, but if it does, it could be misled by this value.