query: Internal Server Error unknown targetHealth: "unknown" when trying to open the targets page

[tizki]

Thanos, Prometheus and Golang version used:
thanos query 0.30.2
thanos sidecar 0.31.0
prometheus 2.45.0

Object Storage Provider:

What happened:
when opening the targets page on thanos query, there is an error: Error: Error fetching Targets information: Internal Server Error
and the targets are not displayed.

in the browsers' developer tools, I can see a call to
api/v1/targets?state=active

when trying to call that endpoint directly I get:

{
_status: "error",
errorType: "internal",
error: "retrieving targets: proxy Targets: receiving targets from targets client &{0xc0009b1400}: rpc error: code = Internal desc = unknown targetHealth: "unknown""
}

What you expected to happen:
targets page shows all targets and no errors.

How to reproduce it (as minimally and precisely as possible):

Full logs to relevant components:

Anything else we need to know:
as discussed at https://cloud-native.slack.com/archives/CK5RSSC10/p1711881117050809
it seems that the unknow state is missing for the TargetHealth_value at the targetpb/rpc.proto