fix: only enable full scan when enable_full_scan is set explicitly forleast request lb

[wbpcode]

Commit Message: fix: only enable full scan when enable_full_scan is set explicitly forleast request lb
Additional Description:

#29873 introduced a minor breaking change. The full scan will be enabled automatically when host num is less than choice count.
Although from technical point, I think this change basically harmless and could make a better load balancing result, a new runtime guard should be enough.

But in practice, it effects our downstream users in a unexpected way. See #30768 (comment)

No change log is updated or added because our change log or doc doesn't mention that full scan will be enabled automatically when host num less than choice count.

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Testing:
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
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[wbpcode]

cc @lizan could you take a look as a senior maintainer when you have free time? Thanks.

[wbpcode]

cc @barroca

[ggreenway]

@barroca part of your change had to be reverted due to a regression. I think this can be fixed by, during the full scan, picking a random host from those that have the lowest rq count, instead of the first host with that count. Would you be willing to make that fix?

[tomwans]

@barroca part of your change had to be reverted due to a regression. I think this can be fixed by, during the full scan, picking a random host from those that have the lowest rq count, instead of the first host with that count. Would you be willing to make that fix?

To be explicit here: if active_rq is equal across all hosts, and the host list is in the same order for all callers, you can end up picking the same host in the list every time if active_rq is the same for all of them. This can happen if you have a Envoy cluster that is rarely called but responds with low latency, so active_rq tends to be zero across all hosts. Maybe we start the for loop from a random index when doing the full scan, and that should alleviate this issue as well?