Allow disabling of stacksampling with an env var

This gives an easy way to do so across all services without adding new options.

Reasons to do so might be to avoid overheads or because your prometheus metrics grow too large.

common/common/stats.py

@@ -194,6 +194,10 @@ def install_stacksampler(interval=0.005):
 	We could use user+sys time but that leads to interrupting syscalls,
 	which may affect performance, and we care mostly about user time anyway.
 	"""
+	if os.environ.get('WUBLOADER_DISABLE_STACKSAMPLER', '').lower() == 'true':
+		logging.info("Not installing stacksampler - disabled by WUBLOADER_DISABLE_STACKSAMPLER env var")
+		return
+
 	# Note we only start each next timer once the previous timer signal has been processed.
 	# There are two reasons for this:
 	# 1. Avoid handling a signal while already handling a signal, however unlikely,