Consolidate all COS objects in one location (#330)

## Issue
We are instantiating `TracingEndpointRequirer` in the
`abstract_charm.py` file, but the remaining COS objects in
`relations/cos.py`

## Solution
Move instantiation of `TracingEndpointRequirer` in `relations/cos.py`

Author: shayancanonical

src/abstract_charm.py

@@ -8,7 +8,6 @@
 import typing
 
 import ops
-from charms.tempo_coordinator_k8s.v0.tracing import TracingEndpointRequirer
 
 import container
 import lifecycle
@@ -33,9 +32,6 @@ class MySQLRouterCharm(ops.CharmBase, abc.ABC):
     _READ_WRITE_X_PORT = 6448
     _READ_ONLY_X_PORT = 6449
 
-    _TRACING_RELATION_NAME = "tracing"
-    _TRACING_PROTOCOL = "otlp_http"
-
     def __init__(self, *args) -> None:
         super().__init__(*args)
         # Instantiate before registering other event observers
@@ -71,10 +67,6 @@ def __init__(self, *args) -> None:
         )
         self.tls = relations.tls.RelationEndpoint(self)
 
-        self.tracing = TracingEndpointRequirer(
-            self, relation_name=self._TRACING_RELATION_NAME, protocols=[self._TRACING_PROTOCOL]
-        )
-
     @property
     @abc.abstractmethod
     def _subordinate_relation_endpoint_names(self) -> typing.Optional[typing.Iterable[str]]:
@@ -159,8 +151,7 @@ def _tls_certificate(self) -> typing.Optional[str]:
     @property
     def tracing_endpoint(self) -> typing.Optional[str]:
         """Otlp http endpoint for charm instrumentation."""
-        if self.tracing.is_ready():
-            return self.tracing.get_endpoint(self._TRACING_PROTOCOL)
+        return self._cos_relation.tracing_endpoint
 
     def _cos_exporter_config(self, event) -> typing.Optional[relations.cos.ExporterConfig]:
         """Returns the exporter config for MySQLRouter exporter if cos relation exists"""

src/relations/cos.py

@@ -11,6 +11,7 @@
 from charms.grafana_k8s.v0.grafana_dashboard import GrafanaDashboardProvider
 from charms.loki_k8s.v1.loki_push_api import LogProxyConsumer
 from charms.prometheus_k8s.v0.prometheus_scrape import MetricsEndpointProvider
+from charms.tempo_coordinator_k8s.v0.tracing import TracingEndpointRequirer
 
 import container
 import relations.secrets
@@ -46,6 +47,9 @@ class COSRelation:
     MONITORING_USERNAME = "monitoring"
     _MONITORING_PASSWORD_KEY = "monitoring-password"
 
+    _TRACING_RELATION_NAME = "tracing"
+    _TRACING_PROTOCOL = "otlp_http"
+
     def __init__(self, charm_: "abstract_charm.MySQLRouterCharm", container_: container.Container):
         self._grafana_dashboards = GrafanaDashboardProvider(charm_)
         self._metrics_endpoint = MetricsEndpointProvider(
@@ -62,6 +66,9 @@ def __init__(self, charm_: "abstract_charm.MySQLRouterCharm", container_: contai
                 },
             },
         )
+        self._tracing = TracingEndpointRequirer(
+            charm_, relation_name=self._TRACING_RELATION_NAME, protocols=[self._TRACING_PROTOCOL]
+        )
 
         self._charm = charm_
         self._container = container_
@@ -91,6 +98,12 @@ def exporter_user_config(self) -> ExporterConfig:
             listen_port=self._EXPORTER_PORT,
         )
 
+    @property
+    def tracing_endpoint(self) -> typing.Optional[str]:
+        """The tracing endpoint."""
+        if self._tracing.is_ready():
+            return self._tracing.get_endpoint(self._TRACING_PROTOCOL)
+
     @property
     def relation_exists(self) -> bool:
         """Whether relation with cos exists."""