Unreal Engine: Add event fingerprint API docs (#13808)

This PR adds docs for the new event class API that allows get/set its
fingerprint. Corresponding changes were introduced to Unreal SDK in
scope of getsentry/sentry-unreal#920.

Closes #13733

---------

Co-authored-by: Shannon Anahata <[email protected]>

Author: 2 people

docs/platforms/unreal/usage/sdk-fingerprinting/index.mdx

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+---
+title: "SDK Fingerprinting"
+description: "Learn about overriding default fingerprinting in your SDK."
+sidebar_order: 30
+---
+
+All events have a fingerprint. Events with the same fingerprint are grouped together into an issue.
+
+By default, Sentry will run one of our built-in grouping algorithms to generate a fingerprint based on information available within the event such as `stacktrace`, `exception`, and `message`. To extend the default grouping behavior or change it completely, you can use a combination of the following options:
+
+1. In your SDK, using SDK Fingerprinting, as documented below
+2. In your project, using [Fingerprint Rules](/concepts/data-management/event-grouping/fingerprint-rules/) or [Stack Trace Rules](/concepts/data-management/event-grouping/stack-trace-rules/)
+
+In supported SDKs, you can override Sentry's default grouping that passes the fingerprint attribute as an array of strings. The length of a fingerprint's array is not restricted. This works similarly to the [fingerprint rules functionality](/concepts/data-management/event-grouping/fingerprint-rules/), which is always available and can achieve similar results.
+
+## Basic Example
+
+In the most basic case, values are passed directly:
+
+<PlatformContent includePath="set-fingerprint/basic" />
+
+You can use variable substitution to fill dynamic values into the fingerprint that by default is computed on the server. For example, the value `{{ default }}` can be added to include the entire normally generated grouping hash into the fingerprint. These values are the same for server-side fingerprinting. See [Variables](/concepts/data-management/event-grouping/fingerprint-rules/#variables) for more information.
+
+## Group Errors With Greater Granularity
+
+In some scenarios, you'll want to group errors more granularly.
+
+For example, if your application queries a Remote Procedure Call Model (RPC) interface or external Application Programming Interface (API) service, the stack trace is generally the same, even if the outgoing request is very different.
+
+The following example will split up the default group Sentry would create (represented by `{{ default }}`) even more, taking some attributes on the error object into account:
+
+<PlatformContent includePath="set-fingerprint/rpc" />
+
+## Group Errors More Aggressively
+
+You can also overwrite Sentry's grouping entirely.
+
+For example, if a generic error, such as a database connection error, has many different stack traces and never groups them together, you can overwrite Sentry's grouping by omitting `{{ default }}` from the array:
+
+<PlatformContent includePath="set-fingerprint/database-connection" />

platform-includes/set-fingerprint/basic/unreal.mdx

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+```cpp
+FHttpModule* Http = &FHttpModule::Get();
+TSharedRef<IHttpRequest, ESPMode::ThreadSafe> HttpRequest = Http->CreateRequest();
+
+// Configure the request
+HttpRequest->SetURL(URL);
+HttpRequest->SetVerb("POST");
+HttpRequest->SetHeader(TEXT("Content-Type"), TEXT("application/json"));
+HttpRequest->SetContentAsString(JsonString);
+HttpRequest->OnProcessRequestComplete().BindLambda([=](FHttpRequestPtr Request, FHttpResponsePtr Response, bool bWasSuccessful)
+{
+    if (bWasSuccessful && Response.IsValid() && Response->GetResponseCode() != 200)
+    {
+        // Handle successful response
+    }
+    else
+    {
+        TArray<FString> Fingerprint;
+        Fingerprint.Add("http-request-error");
+        Fingerprint.Add(Request->GetURL());
+        Fingerprint.Add(Request->GetVerb());
+
+        // Add more context if needed
+
+        USentryEvent* Event = NewObject<USentryEvent>();
+        Event->Initialize();
+        Event->SetFingerprint(Fingerprint);
+
+        USentrySubsystem* SentrySubsystem = GEngine->GetEngineSubsystem<USentrySubsystem>();
+        SentrySubsystem->CaptureEvent(Event, ESentryLevel::Error);
+    }
+});
+```

platform-includes/set-fingerprint/database-connection/unreal.mdx

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+```cpp
+USentryEvent* Event = NewObject<USentryEvent>();
+Event->Initialize();
+Event->SetFingerprint( { "database-connection-error" } );
+
+USentrySubsystem* SentrySubsystem = GEngine->GetEngineSubsystem<USentrySubsystem>();
+SentrySubsystem->CaptureEvent(Event, ESentryLevel::Error);
+```

platform-includes/set-fingerprint/rpc/unreal.mdx

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+```cpp
+USTRUCT()
+struct FSomeRpcError
+{
+    GENERATED_BODY()
+
+    UPROPERTY()
+    FString Function;
+    UPROPERTY()
+    FString Code;
+}
+
+// Some code that emits RPC error info
+FSomeRpcError RpcError = /* ... */
+
+USentryEvent* Event = NewObject<USentryEvent>();
+Event->Initialize();
+Event->SetFingerprint( { "{{ default }}", RpcError.Function, RpcError.Code } );
+
+USentrySubsystem* SentrySubsystem = GEngine->GetEngineSubsystem<USentrySubsystem>();
+SentrySubsystem->CaptureEvent(Event, ESentryLevel::Error);
+```