add transaction/span setting docs for Go

Author: giortzisg

docs/platforms/go/common/tracing/instrumentation/custom-instrumentation/index.mdx

@@ -15,3 +15,70 @@ To capture transactions and spans customized to your organization's needs, you m
 <PlatformContent includePath="performance/add-spans-example" />
 
 <PlatformContent includePath="performance/retrieve-transaction" />
+
+## Adding Span & Transaction Data Attributes
+
+You can capture data attributes along with your spans and transactions. You can specify data attributes when starting a span or transaction:
+
+```go
+		http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+			ctx := r.Context()
+
+      options := []sentry.SpanOption{
+        // Set the OP based on values from https://develop.sentry.dev/sdk/performance/span-operations/
+        sentry.WithOpName("http.server"),
+        sentry.ContinueFromRequest(r),
+        sentry.WithTransactionSource(sentry.SourceURL),
+      }
+
+		  // Create a transaction and assign data attributes
+			transaction := sentry.StartTransaction(ctx,
+				fmt.Sprintf("%s %s", r.Method, r.URL.Path),
+				options...,
+			)
+			transaction.SetData("dataAttr1", 42)
+			transaction.SetData("dataAttr2", true)
+			// omitted code ...
+			transaction.Finish()
+
+			// ... or create a span and assign data attributes
+			span := sentry.StartSpan(ctx, "span1")
+			span.SetData("dataAttr1", 42)
+			span.SetData("dataAttr2", true)
+			// omitted code ...
+			span.Finish()
+		})
+```
+
+Or you can add data attributes to an existing transaction or span:
+
+```go
+		http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+			ctx := r.Context()
+			transaction := sentry.TransactionFromContext(ctx)
+
+			if transaction != nil {
+				transaction.SetData("dataAttr1", 42)
+				transaction.SetData("dataAttr2", true)
+			}
+
+			span := sentry.SpanFromContext(ctx)
+			span.SetData("dataAttr1", 42)
+			span.SetData("dataAttr2", true)
+		})
+```
+
+To attach data attributes to the transaction and all its spans, you can use `BeforeSendTransaction`:
+
+```go
+    sentry.Init(sentry.ClientOptions{
+      Dsn: "___PUBLIC_DSN___",
+      BeforeSendTransaction: func(event *sentry.Event, hint *sentry.EventHint) *sentry.Event {
+        for _, sp := range event.Spans {
+          sp.SetData("dataAttr1", 42)
+          sp.SetData("dataAttr2", true)
+        }
+        return event
+      },
+    })
+```