Add how to connect to Azure using Service Principal (#7485)

Add documentation for https://github.com/grafana/loki/pull/7179 which
added authentication using a Service Principal for Azure.

docs/sources/storage/_index.md

@@ -36,6 +36,11 @@ The file system is the simplest backend for chunks, although it's also susceptib
 
 S3 is AWS's hosted object store. It is a good candidate for a managed object store, especially when you're already running on AWS, and is production safe.
 
+### Azure Blob Storage
+
+Blob Storage is Microsoft Azure's hosted object store. It is a good candidate for a managed object store, especially when you're already running on Azure, and is production safe.
+You can authenticate Blob Storage access by using a storage account name and key or by using a Service Principal.
+
 ### Notable Mentions
 
 You may use any substitutable services, such as those that implement the S3 API like [MinIO](https://min.io/).
@@ -291,6 +296,8 @@ schema_config:
 
 ### Azure Storage Account
 
+#### Using account name and key
+
 ```yaml
 schema_config:
   configs:
@@ -323,3 +330,36 @@ storage_config:
   filesystem:
     directory: /data/loki/chunks
 ```
+
+#### Using a service principal
+
+```yaml
+schema_config:
+  configs:
+  - from: "2020-12-11"
+    index:
+      period: 24h
+      prefix: index_
+    object_store: azure
+    schema: v11
+    store: boltdb-shipper
+storage_config:
+  azure:
+    use_service_principal: true
+    # Azure tenant ID used to authenticate through Azure OAuth
+    tenant_id : <tenant-id>
+    # Azure Service Principal ID
+    client_id: <client-id>
+    # Azure Service Principal secret key
+    client_secret: <client-secret>
+    # See https://docs.microsoft.com/en-us/azure/storage/blobs/storage-blobs-introduction#containers
+    container_name: <container-name>
+    request_timeout: 0
+  boltdb_shipper:
+    active_index_directory: /data/loki/boltdb-shipper-active
+    cache_location: /data/loki/boltdb-shipper-cache
+    cache_ttl: 24h
+    shared_store: azure
+  filesystem:
+    directory: /data/loki/chunks
+```