feat: receive-otlp relation for OTLP logs

[MichaelThamm]

Checklist

• PR title makes an appropriate release note and follows conventional commits syntax.
• Merge target is the correct branch, and relevant tandem backport PRs opened.
• mention to IS about the potential for scaled otelcol not loadbalancing with ingress. Let us know if so

Issue

We need a receive-otlp integration so we can receive OTLP LogRecords.

Solution

Loki supports logs at LOKI:3100/otlp.

Building on the work (AKA copying over the code) done in:

• feat: send/receive relations for forwarding OTLP metrics  opentelemetry-collector-k8s-operator#131

requires these changes to Loki:

1. Update the nginx.conf so that

location = /otlp/v1/logs {
    set $backend http://write;
    proxy_pass $backend;
    proxy_connect_timeout 5s;
}

2. Implement the library

self._otlp = OtlpProvider(self)
self._otlp.add_endpoint(ProtocolType.http, f"{self.external_url}/otlp", [TelemetryType.logs])
self._otlp.publish()

Testing Instructions

1. terraform apply Loki standalone

terraform {
  required_version = ">= 1.5"
  required_providers {
    juju = {
      source  = "juju/juju"
      version = "~> 1.0"
    }
  }
}

resource "juju_model" "coordinated-worker" {
  name = "loki-standalone"
}

module "loki" {
  source            = "git::https://github.com/canonical/observability-stack//terraform/loki"
  model_uuid        = juju_model.coordinated-worker.uuid
  channel           = "dev/edge"
  s3_endpoint       = "http://S3_ENDPOINT:8333" # Seaweed
  s3_secret_key     = "placeholder"
  s3_access_key     = "placeholder"
  coordinator_units = 1
  backend_units     = 1
  read_units        = 1
  write_units       = 1
}

2. charmcraft pack loki-coordinator
3. juju refresh loki --path ./loki-coordinator-k8s_ubuntu@24.04-amd64.charm
4. jd opentelemetry-collector-k8s --trust --channel "dev/edge" otelcol (assuming it has the OTLP feature)
5. jd traefik-k8s traefik --trust --channel latest/edge
6. jd grafana-k8s grafana --trust --channel dev/edge
7. jrel otelcol loki:receive-otlp
8. jrel traefik otelcol:ingress
9. jrel grafana loki:grafana-source
10. Check logs in Grafana from opentelemetry-collector charm