traefik looks for otlp_grpc tracing endpoint but has not requested it

[PietroPasotti]

Bug Description

When relating tempo to traefik, if tempo does not reply with an otlp_http endpoint, traefik will look for a grpc one.
However that's not been requested in init and so that raises an exception.

To Reproduce

juju deploy tempo
juju deploy traefik
jhack imatrix fill

boom

Environment

n/a

Relevant log output

[...snip]



 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/venv/ops/framework.py", line 865, in _emit
   self._reemit(event_path)
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/venv/ops/framework.py", line 955, in _reemit
   custom_handler(event)
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/lib/charms/tempo_k8s/v2/tracing.py", line 780, in _on_tracing_relation_changed
   self.on.endpoint_changed.emit(relation, data.host, [i.dict() for i in data.receivers])  # type: ignore
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/venv/ops/framework.py", line 352, in emit
   framework._emit(event)
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/venv/ops/framework.py", line 865, in _emit
   self._reemit(event_path)
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/venv/ops/framework.py", line 955, in _reemit
   custom_handler(event)
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/lib/charms/tempo_k8s/v1/charm_tracing.py", line 532, in wrapped_function
   return callable(*args, **kwargs)  # type: ignore
 File "./src/charm.py", line 370, in _on_tracing_endpoint_changed
   self._configure_tracing()
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/lib/charms/tempo_k8s/v1/charm_tracing.py", line 532, in wrapped_function
   return callable(*args, **kwargs)  # type: ignore
 File "./src/charm.py", line 436, in _configure_tracing
   elif endpoint := self._tracing.otlp_grpc_endpoint():
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/lib/charms/tempo_k8s/v2/tracing.py", line 838, in otlp_grpc_endpoint
   return self.get_endpoint("otlp_grpc")
 File "/var/lib/juju/agents/unit-traefik-k8s-0/charm/lib/charms/tempo_k8s/v2/tracing.py", line 826, in get_endpoint
   raise ProtocolNotRequestedError(protocol, relation)
harms.tempo_k8s.v2.tracing.ProtocolNotRequestedError: ('otlp_grpc', <ops.model.Relation tracing:42>)

Additional context

No response

[PietroPasotti]

solved by #299