Base docker-compose

[olegbespalov]

What?

This PR adds basic docker-compose for local testing and developing the k6 opentelemetry output.

It contains:

• Grafana Alloy has gRPC and HTTP receivers for OpenTelemetry metrics
• Prometheus as the actual metrics backend
• Grafana + Grafana dashboards, copied from the prometheus remote write extension

Why?

Having this as the docker-compose helps during the development and testing of the open telemetry output. It also reduces the scope of the following PRs.

Part of: #1