Unreal Engine: Update sampling docs (#14740)

This PR updates `Sampling` section for Unreal SDK docs:
- Removed notice that trace sampling function is not supported on
Windows and Linux
- Updated custom trace sampler example

Related to:
- getsentry/sentry-unreal#1057

Author: tustanivsky

docs/platforms/unreal/configuration/options.mdx

@@ -186,10 +186,4 @@ A number between `0` and `1`, controlling the percentage chance a given transact
 
 A function responsible for determining the percentage chance a given transaction will be sent to Sentry. It will automatically be passed information about the transaction and the context in which it's being created, and must return a number between `0` (0% chance of being sent) and `1` (100% chance of being sent). Can also be used for filtering transactions, by returning 0 for those that are unwanted. Either this or <PlatformIdentifier name="traces-sample-rate" /> must be defined to enable tracing.
 
-<Alert>
-
-Currently, there is no support for sampling functions on Windows or Linux (<PlatformIdentifier name="traces-sampler" />).
-
-</Alert>
-
 </ConfigKey>

docs/platforms/unreal/tracing/index.mdx

@@ -22,12 +22,6 @@ First, enable tracing and configure the sample rate for transactions. Set the sa
 
 The two options are meant to be mutually exclusive. If you set both, <PlatformIdentifier name="traces-sampler" /> will take precedence.
 
-<Alert>
-
-Currently there is no support for sampling functions on Windows/Linux (<PlatformIdentifier name="traces-sampler" />).
-
-</Alert>
-
 <PlatformContent includePath="performance/configure-sample-rate" />
 
 Learn more about tracing <PlatformLink to="/configuration/options/#tracing-options">options</PlatformLink>, how to use the <PlatformLink to="/configuration/sampling/#setting-a-sampling-function">TracesSampler</PlatformLink> function, or how to <PlatformLink to="/configuration/sampling/#sampling-transaction-events">sample transactions</PlatformLink>.

platform-includes/performance/traces-sampler-as-sampler/unreal.mdx

@@ -7,20 +7,20 @@ class USomeTraceSampler : public USentryTraceSampler
 public:
 	virtual bool Sample_Implementation(USentrySamplingContext* samplingContext, float& samplingValue) override
     {
-        const FString& gameMode =
+        const FSentryVariant& gameMode =
             *samplingContext->GetCustomSamplingContext().Find("GameMode");
 
-        if (gameMode.Equals(TEXT("ranked")))
+        if (gameMode.GetValue<FString>().Equals(TEXT("ranked")))
         {
             // Ranked matches are important - take a big sample
             samplingValue = 0.5;
         }
-        else if (gameMode.Equals(TEXT("quick_match")))
+        else if (gameMode.GetValue<FString>().Equals(TEXT("quick_match")))
         {
             // Quick matches less important and happen more frequently - only take 20%
             samplingValue = 0.2;
         }
-        else if (gameMode.Equals(TEXT("training")))
+        else if (gameMode.GetValue<FString>().Equals(TEXT("training")))
         {
             // Training matches are just noise - drop all transactions
             samplingValue = 0.0;