refactor: Improve testing and docs

[MichaelThamm]

Context

Why is this needed?

This PR simplifies the OTLP testing by removing the otlp_dual_ctx charm fixture for testing the requirer and provider charms in isolation. This is the intended way to test libraries with Scenario since we can provide remote and local databags to provide sufficient coverage. The previous testing was leaking otelcol implementation detail into the library.

Drive-by fixes

• Make the _OtlpAppData models private since they are not public and to indicate that they are not intended to be.
• We were missing some coverage for single and official rule file formats
• Since we removed the leaked charm implementation from the lib and the otlp_dual_ctx charm fixture, tests for metadata and provider rules were added.
• Fix the metadata in tests and in documentation to use juju_charm instead of charm.
• Replace old-style charming practices (when we wrote observe-pattern libs) with reconciler-style charming patterns in the README, __init__, and conftest.

For example, before:

class MyOtlpServer(CharmBase):
    def __init__(self, *args):
        super().__init__(*args)
        self.otlp_provider = OtlpProvider(self)
        self.framework.observe(self.on.ingress_ready, self._on_ingress_ready)

    def _on_ingress_ready(self, event):
        self.otlp_provider.add_endpoint(
            protocol="grpc",
            endpoint="https://my-app.ingress:4317",
            telemetries=["logs", "metrics"],
        )
        self.otlp_provider.add_endpoint(
            protocol="http",
            endpoint="https://my-app.ingress:4318",
            telemetries=["traces"],
        )
        self.otlp_provider.publish()

now:

class MyOtlpServer(CharmBase):
    def __init__(self, *args):
        super().__init__(*args)
        self.framework.observe(self.on.ingress_ready, self._on_ingress_ready)

    def _on_ingress_ready(self, event):
        OtlpProvider(self).add_endpoint(
            protocol="grpc",
            endpoint="https://my-app.ingress:4317",
            telemetries=["logs", "metrics"],
        ).add_endpoint(
            protocol="http",
            endpoint="https://my-app.ingress:4318",
            telemetries=["traces"],
        ).publish()

User-facing changes

This PR does not actually functionally change the lib and is simply cosmetic. However, the documentation was updated which is user-facing. I will release as part of:

• feat: Replace the requirer's rule interface with a rules object #376