Whether something is "in code" or not is orthogonal to the problem of tracking infrastructure changes. E.G. You can commit a change to Terraform, but that doesn't tell someone when "terraform apply" occurred.

What you should use depends on what tools you have available. We currently use Datadog, so we use their API for publishing events. We can then search for them in the event stream or overlay them on dashboards.

You could look at the datastores Grafana supports for annotations and choose whichever of them you're comfortable operating.

http://docs.grafana.org/reference/annotations/