Add span.kind=server to aws.lambda span

This aligns the tracer-created span with the extension's behavior,
enabling server-based features in Datadog APM for Lambda functions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jchrostek-dd

datadog_lambda/tracing.py

@@ -1460,6 +1460,7 @@ def create_function_execution_span(
         function_arn = ":".join(tk[0:7]) if len(tk) > 7 else function_arn
         function_version = tk[7] if len(tk) > 7 else "$LATEST"
         tags = {
+            "span.kind": "server",
             "cold_start": str(is_cold_start).lower(),
             "function_arn": function_arn,
             "function_version": function_version,