This patch adds a new /agent/v1/metrics that will return metrics on the
running Vault agent. Configuration is done using the same telemetry
stanza as the Vault server. For now default runtime metrics are
returned with a few additional ones specific to the agent:
- `vault.agent.auth.failure` and `vault.agent.auth.success` to monitor
the correct behavior of the auto auth mechanism
- `vault.agent.proxy.success`, `vault.agent.proxy.client_error` and
`vault.agent.proxy.error` to check the connection with the Vault server
- `vault.agent.cache.hit` and `vault.agent.cache.miss` to monitor the
cache
Closes https://github.com/hashicorp/vault/issues/8649
Co-authored-by: Theron Voran <tvoran@users.noreply.github.com>
