cold starts: basebackup compression (#4482)

Co-authored-by: Alex Chi Z <iskyzh@gmail.com>

test_runner/performance/test_startup.py

@@ -60,6 +60,11 @@ def test_startup_simple(neon_env_builder: NeonEnvBuilder, zenbenchmark: NeonBenc
             value = metrics[key]
             zenbenchmark.record(name, value, "ms", report=MetricReport.LOWER_IS_BETTER)
 
+        # Check basebackup size makes sense
+        basebackup_bytes = metrics["basebackup_bytes"]
+        if i > 0:
+            assert basebackup_bytes < 100 * 1024
+
         # Stop so we can restart
         endpoint.stop()