prometheus-rules relation fails with missing template

[Xav Paice]

When running the prometheus-rules-relation-joined hook, I see the following traceback:

2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 Traceback (most recent call last):
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/charm/hooks/prometheus-rules-relation-joined", line 22, in <module>
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 main()
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.5/site-packages/charms/reactive/__init__.py", line 74, in main
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 bus.dispatch(restricted=restricted_mode)
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.5/site-packages/charms/reactive/bus.py", line 390, in dispatch
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 _invoke(other_handlers)
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.5/site-packages/charms/reactive/bus.py", line 359, in _invoke
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 handler.invoke()
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.5/site-packages/charms/reactive/bus.py", line 181, in invoke
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 self._action(*args)
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/charm/reactive/telegraf.py", line 1291, in render_prometheus_rules
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 with open(os.path.join(get_templates_dir(), template_file), "r") as fd:
2021-01-27 21:34:06 WARNING unit.telegraf/0.prometheus-rules-relation-joined logger.go:60 FileNotFoundError: [Errno 2] No such file or directory: '/var/lib/juju/agents/unit-telegraf-0/charm/templates/rule_cpu_usage.tmpl'
2021-01-27 21:34:06 ERROR juju.worker.uniter.operation runhook.go:136 hook "prometheus-rules-relation-joined" (via explicit, bespoke hook script) failed: exit status 1

The actual file name in the charm is rule_cpu_usage.j2 not rule_cpu_usage.tmpl.