Alert metadata is omitted in several alerts

[daniel-anova]

Describe the bug

It's possible to specify the clusterName to distinguish which cluster is relaying a message when several clusters report to the same channel.

However, most messages like completion and errors seem to be hardcoded to not include that metadata which contains said cluster name making the origin of those messages unknown.

To Reproduce

Deploy flagger chart with clusterName set and an AlertProvider configured

Expected behavior

All notifications would have clusterName metadata to identify their origin.

Additional context

• Flagger version: 1.21
• Kubernetes version: 1.23.8
• Service Mesh provider: istio 1.14.1
• Ingress provider: istio 1.14.1