Add ingress relation to otelcol and cos tf modules so we can send-otlp when COS is ingressed

When we relate 2 otelcol charms - one VM sending OTLP to one in the COS model. This fails if COS has ingress. This is solved if we add otelcol to the [ingress input in COS TF](https://github.com/canonical/observability-stack/blob/882110dca451cb9ba9dba23782cc6bda49ee17a9/terraform/cos/variables.tf#L76).