Telemetry option disable_hostname always true when only one sink is used

[verdel]

Describe the bug

If disable_hostname is set to false in the telemetry configuration and only a single sink is configured (for example, Prometheus), metric names do not include the hostname prefix.

If enable_hostname_label is set to true, the host label containing the hostname appears in the metrics.

This behavior differs from what is described in the documentation.

To Reproduce
Steps to reproduce the behavior:

1. Run vault with telemetry.disable_hostname=false in configuration
2. Get prometheus metrics from http://localhost:8200/v1/sys/metrics?format=prometheus
3. Observe that metric names do not contain a hostname prefix, even though telemetry.disable_hostname is set to false.

Expected behavior

The presence of a hostname prefix in metric names when the telemetry.disable_hostname option is set to false.

Environment:

• Vault Server Version (retrieve with vault status): 1.21.2
• Vault CLI Version (retrieve with vault version): 1.21.2
• Server Operating System/Architecture: Ubuntu 24.04 x86_64

Vault server configuration file(s):

api_addr     = "http://localhost:8200"

storage "file" {
  path    = "/vault/data"
}

listener "tcp" {
  address     = "0.0.0.0:8200"
  tls_disable = 1
  telemetry {
    unauthenticated_metrics_access = true
  }
}

telemetry {
  disable_hostname = false
}

disable_mlock = true

Additional context

Most likely, the issue is related to this part of the code. If the number of configured sinks is less than 2, EnableHostname is set to false, which is equivalent to the behavior when telemetry.disable_hostname=true is set in the configuration.

Either the condition needs to be changed to > 0, or this code should be moved slightly lower, after the inm is added to the fanout.