Receive external labels proposal

Signed-off-by: haanhvu <haanh6594@gmail.com>

docs/proposals-accepted/202209-receive-tenant-external-labels.md

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+---
+type: proposal
+title: Allow specifying tenant-specific external labels in Receivers
+status: accepted
+owner: haanhvu
+menu: proposals-accepted
+---
+
+## 1 Related links/tickets
+
+* https://github.com/thanos-io/thanos/issues/5434
+
+## 2 What problem(s) are we trying to solve?
+
+We want to add arbitrary external labels to each tenant in Thanos Receiver.
+
+## 3 How is it going to be helpful?
+
+This new functionality is helpful in many use cases. Two typical ones are:
+* Compact tenant blocks based on tenants’ external labels
+* Query data from tenants which share the same external labels
+
+## 4 Pitfalls of the current solution
+
+Currently, we can only add external labels to Receivers. Each tenant in a Receiver only gets one external label, which is the tenant ID. This is not flexible enough to do complex compact and query works at the tenant level.
+
+## 5 What are the possible solution approaches?
+
+We’re seeing two solution approaches:
+* Specify the tenants for each external label in Receiver: Since we already have the functionality of adding external labels to Receiver, is there a way to specify which tenants each external label goes to? Because many tenants may share the same external labels, this may be a straightforward way for users.
+* Add external labels to each tenant directly: This way, we treat each tenant independently, which seems to be a more straightforward way in implementation. If goinf this way, we can start with the tenant configuration file.
+
+## 6 Which solution approach is the most favorable?
+
+After discussing internally in the issue team (@fpetkovski, @saswatamcode, @haanhvu), we agree the second approach may be the better one to start with since it’s more straightforward in implementation.
+
+## 7 Potential challenges of the favorable approach?
+
+We’re seeing two main challenges:
+* Support external labels for decoupled routers and ingesters: Receive routers and ingestors are decoupled functionally, but currently they are not decoupled in code. Issue [#5643](https://github.com/thanos-io/thanos/issues/5643) has already been raised for this. With routers and ingestors intermingled in code, we need to find a way to support external labels for RouterOnly and IngestorOnly. One apparent way is to fix [#5643](https://github.com/thanos-io/thanos/issues/5643) at least partially enough before tackling this challenge.
+* Handling changes for external labels
+
+## 8 Implementation plan?
+
+* RouterIngestor:
+  - Add a new `external_labels` field in `tenant` field in hasring config
+  - Map tenant's `external_labels` to tenant's `tenant_id`
+  - Wire tenant’s `external_labels` to the tenant’s TSDB instance
+* RouterOnly
+* IngestorOnly
+
+### 9 Test plan?