add checkpointStrategy parameter to event-hub (#429)

content/docs/2.3/scalers/azure-event-hub.md

@@ -4,7 +4,6 @@ layout = "scaler"
 availability = "v1.0+"
 maintainer = "Community"
 description = "Scale applications based on Azure Event Hubs."
-notice = "As of now, the Event Hub scaler only supports reading checkpoints from Blob Storage, as well as scaling only Event Hub applications written in C#, Java, Python or created with Azure Functions."
 go_file = "azure_eventhub_scaler"
 +++
 
@@ -27,9 +26,18 @@ triggers:
 
 - `connectionFromEnv` - Name of the environment variable your deployment uses to get the connection string appended with `EntityPath=<event_hub_name>`
 - `storageConnectionFromEnv` - Name of the environment variable that provides connection string for Azure Storage Account to store checkpoint. As of now the Event Hub scaler only reads from Azure Blob Storage.
-- `consumerGroup` - Consumer group of event hub consumer. (default: `$default`)
+- `consumerGroup` - Consumer group of Azure Event Hub consumer. (default: `$default`)
 - `unprocessedEventThreshold` - : Average target value to trigger scaling actions. (default: 64)
-- `blobContainer` - Container name to store checkpoint. This is needed when a using an Event Hub application written in dotnet or java, and not an Azure function
+- `blobContainer` - Container name to store checkpoint. This is needed for every `checkpointStrategy` except of `AzureFunction`. With Azure Functions the `blobContainer` is autogenerated and cannot be overridden.
+- `checkpointStrategy` - configure the checkpoint behaviour of different Event Hub SDKs. (default: `""`)
+    - `azureFunction`: Suitable for Azure Functions & Azure WebJobs SDK. This is the default setting, when `blobcontainer` is not specified.
+    - `blobMetadata`: For all implementations that store checkpoint informations on blob metadata such as current C#, Python, Java and JavaScript Event Hub SDKs.
+    - `goSdk` For all implementations using the [Golang SDK](https://github.com/Azure/azure-event-hubs-go)'s checkpointing, for example Dapr
+    - When no checkpoint strategy is specified, the Event Hub scaler will use backwards compatibility and able to scale older implementations of C#, Python or Java Event Hub SDKs. (see "Legacy checkpointing"). If this behaviour should be used, `blobContainer` is also required.
+
+> 💡 **Legacy Checkpointing:** C# applications, which use the `Microsoft.Azure.EventHubs` package, Java applications which use the `azure-eventhubs-eph` package or Python applications which use the `azure-eventhub` below `v5` are supported if no `checkpointStrategy` is specified. These legacy implementations are based on the `EventProcessorHost` client, which stores checkpoint information as blob container content
+> 
+> C# Applications that use the current `Azure.Messaging.EventHubs` package, Java applications which use the current `azure-messaging-eventhubs` package or Python applications since `azure-eventhub v5`, have to set the `checkpointStrategy`to `blobMetadata`. All these implementations based on the `EventProcessorClient` which stores checkpoint information as blob metadata.
 
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