feat(ruby): Add snippet for adding data to transaction and all its spans (#12559)

Author: sl0thentr0py

platform-includes/performance/improving-data/ruby.mdx

@@ -18,7 +18,7 @@ end
 
 ### Adding Data Attributes to Spans
 
-You can add data attributes to your spans. This data is visible in the trace explorer in Sentry. Data attributes can be strings, numbers or booleans, as well as (non-mixed) arrays of these types:
+You can add data attributes to your spans the same way, with the same type restrictions as described above.
 
 ```ruby
 Sentry.with_child_span(op: "my-span") do |span|
@@ -31,3 +31,24 @@ Sentry.with_child_span(op: "my-span") do |span|
   span.set_data("my-data-attribute-6", [true, false, true])
 end
 ```
+
+### Adding Data Attributes to All Spans
+
+To attach data attributes to the transaction and all its spans, you can use <PlatformLink to="/configuration/filtering/#using-before-send-transaction">`before_send_transaction`</PlatformLink>:
+
+```ruby
+Sentry.init do |config|
+  # ...
+  config.traces_sample_rate = 1.0
+
+  config.before_send_transaction = lambda do |event, _hint|
+    # Set the data attribute "foo" to "bar" on every span belonging to this transaction event
+    event.spans.each { |span| span.set_data(:foo, "bar")
+
+    # Set the data on the transaction itself, too
+    event.contexts[:trace][:data][:foo] = "bar"
+
+    event
+  end
+end
+```