Resolve Vault `AGENT_INJECT_VAULT_ADDR` issue

From 1.35.0 release notes:

• Vault is in beta and therefore not recommended for operational use. We are still working on a few issues. If you set the extra environment variable AGENT_INJECT_VAULT_ADDR for the Injector ENVs you will encounter a helm install error due to duplicate ENVs. In our testing the Prometheus pod is not being injected with a Vault sidecar and Prometheus is not able to scrape metrics.

We need to investigate why prometheus metrics are no longer working, whether it is related to AGENT_INJECT_VAULT_ADDR or not, and update the production docs as needed.