feat(go): Add BeforeSendTransaction example snippet

src/platform-includes/configuration/before-send-transaction/go.mdx

@@ -0,0 +1,15 @@
+The `BeforeSendTransaction` hook is a function that can be passed to `sentry.Init` via the corresponding configuration option. In this function you can either update the transaction event before it's sent to Sentry, or drop it by returning `nil`, for example:
+
+```go
+sentry.Init(sentry.ClientOptions{
+	// ...
+	BeforeSendTransaction: func(event *sentry.Event, hint *sentry.EventHint) *sentry.Event {
+		if event.Message == "test-transaction" {
+			// Don't send the transaction to Sentry
+			return nil
+		}
+		// Update the message for every sent transaction
+		event.Message += " [example]"
+		return event
+	},
+```

src/platforms/common/configuration/filtering.mdx

@@ -101,7 +101,7 @@ It also allows you to sample different transactions at different rates.
 
 Learn more about <PlatformLink to="/configuration/sampling/">configuring the sample rate</PlatformLink>.
 
-<PlatformSection supported={["node", "javascript", "php"]} notSupported={["php.symfony"]}>
+<PlatformSection supported={["node", "javascript", "php", "go"]} notSupported={["php.symfony"]}>
 
 ### Using <PlatformIdentifier name="before-send-transaction" />
 

src/platforms/common/configuration/options.mdx

@@ -566,7 +566,7 @@ This function is called with an SDK-specific message or error event object, and
 
 </ConfigKey>
 
-<ConfigKey name="before-send-transaction" supported={["javascript", "node", "php"]} notSupported={["php.symfony"]}>
+<ConfigKey name="before-send-transaction" supported={["javascript", "node", "php", "go"]} notSupported={["php.symfony"]}>
 
 This function is called with an SDK-specific transaction event object, and can return a modified transaction event object, or `null` to skip reporting the event. One way this might be used is for manual PII stripping before sending.