MutableJsonChange can drain ArrayPool with undisposed JsonDocuments

[eerhardt]

See the following code:

azure-sdk-for-net/sdk/core/Azure.Core/src/DynamicData/MutableJsonChange.cs

Line 60 in c4fb4c5

_serializedValue = JsonDocument.Parse(bytes).RootElement;

The JsonDocument returned from JsonDocument.Parse is backed by one or more ArrayPool arrays; those arrays are returned when the instance is disposed. If it's never disposed, the arrays are never returned to the pool. That means that creating but not disposing lots of JsonDocument instances ends up incurring the cost (and contention) of searching the ArrayPool but ends up falling back to allocating anyway.

This is a similar issue to AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet#2178.

We need to be disposing these JsonDocument objects. See these docs.

cc @annelo-msft