docs: Document Thanos Sharding

Signed-off-by: Xiang Dai <764524258@qq.com>

docs/sharing.md

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+---
+title: Sharing
+type: docs
+menu: thanos
+slug: /Sharing.md
+---
+
+# Store and Compactor Sharding
+
+Store and Compactor Sharding is for Long Term Retention Storage.
+
+## Background
+
+Currently all components that read from object store assume that all the operations and functionality should be done based
+on **all** the available blocks that are present in the certain bucket's root directory.
+
+This is in most cases totally fine, however with time and allowance of storing blocks from multiple `Sources` into the same bucket,
+the number of objects in a bucket can grow drastically.
+
+This means that with time you might want to scale out certain components e.g:
+
+* Compactor: Larger number of objects does not matter much, however compactor has to scale (CPU, network) with number of Sources pushing blocks to the object storage.
+
+### Relabelling
+
+Similar to [promtail](https://github.com/grafana/loki/blob/master/docs/promtail.md#scrape-configs) this config will follow native
+[Prometheus relabel-config](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#relabel_config) syntax.
+
+The relabel config defines filtering process done on **every** synchronization with object storage.
+
+We will allow potentially manipulating with several of inputs:
+
+* External labels:
+  * `<name>`
+
+Output:
+
+* If output is empty, drop block.
+
+By default, on empty relabel-config, all external labels are assumed.
+
+Example usages would be:
+
+* Drop blocks which contains external labels cluster=A
+
+```yaml
+- action: drop
+  regex: "A"
+  source_labels:
+  - cluster
+```
+
+* Keep only blocks which contains external labels cluster=A
+```yaml
+- action: keep
+  regex: "A"
+  source_labels:
+  - cluster
+```
+
+By controling which labels should be included in the blocks, we can do sharing.