Allow pre-provisioned throughput (#68)

* Allowing to use provisioned throughput by setting offerThroughput value to "0".

* Round-about way of re-writing this onto current main, but I wanted to keep the original PR commit in the list for credit.

---------

Co-authored-by: Al Baymar <abaymar@aras.com>

docs/CosmosDBSessionStateProviderAsync.md

@@ -22,7 +22,7 @@ Then, register your new provider like so:
 
 2. *cosmosDBAuthKeySettingKey* - The appsetting key name which points to a CosmosDB auth key
 
-3. *offerThroughput* - The offer throughput provisioned for a collection in measurement of Requests-per-Unit in the Azure DocumentDB database service. If the collection provided doesn't exist, the provider will create a collection with this offerThroughput.
+3. *offerThroughput* - The offer throughput provisioned for a collection in measurement of Requests-per-Unit in the Azure DocumentDB database service. If the collection provided doesn't exist, the provider will create a collection with this offerThroughput. If set to "0", collection will be set to use the default throughput of the database.
 
 4. *connectionMode* - Direct | Gateway
 

src/CosmosDBSessionStateProviderAsync/CosmosDBSessionStateProviderAsync.cs

@@ -887,7 +887,7 @@ namespace Microsoft.AspNet.SessionState
                 IndexingPolicy = s_indexNone
             };
 
-            ContainerResponse response = await database.CreateContainerIfNotExistsAsync(containerProperties, s_offerThroughput).ConfigureAwait(false);
+            ContainerResponse response = await database.CreateContainerIfNotExistsAsync(containerProperties, s_offerThroughput > 0 ? s_offerThroughput : (int?)null).ConfigureAwait(false);
 
             if (response?.Resource?.PartitionKeyPath != partitionKeyPath)
             {