Fix dynamic labels not being present on server access audit events

[gabrielcorado]

The SSH session uses the TargetMetadata function to grab server metadata which is then used on audit logging. The issue was that this function only used the labels available at SetLabels (as it relied on the s.labels attribute).

This PR updates it to use a combination of static and dynamic labels (similar to the GetAllLabels function from resources).

Audit logs example

SSH service configuration:

ssh_service:
  enabled: "yes"
  commands:
  - name: OS
    command: [uname]
    period: 1h

Before:

{
  "addr.local": "127.0.0.1:4443",
  "addr.remote": "127.0.0.1:49840",
  "cluster_name": "root.teleport.dev",
  "code": "T2000I",
  "ei": 0,
  "event": "session.start",
  "initial_command": [
    ""
  ],
  "login": "gabrielcorado",
  "namespace": "default",
  "proto": "ssh",
  "server_addr": "[::]:3022",
  "server_hostname": "root.teleport.dev",
  "server_id": "b51ef330-73dd-4719-b8aa-8fb0c1c2dd65",
  "session_recording": "node",
  "sid": "f885497e-b847-4436-b12c-218c6cd3f04f",
  "size": "141:22",
  "time": "2023-09-21T15:44:20.543Z",
  "uid": "0b76a567-7a52-4a02-9c8f-5d546f8429d6",
  "user": "alice"
}

After:

{
  "addr.local": "127.0.0.1:4443",
  "addr.remote": "127.0.0.1:50162",
  "cluster_name": "root.teleport.dev",
  "code": "T2000I",
  "ei": 0,
  "event": "session.start",
  "initial_command": [
    ""
  ],
  "login": "gabrielcorado",
  "namespace": "default",
  "proto": "ssh",
  "server_addr": "[::]:3022",
  "server_hostname": "root.teleport.dev",
  "server_id": "b51ef330-73dd-4719-b8aa-8fb0c1c2dd65",
+ "server_labels": {
+   "OS": "Darwin"
+ },
  "session_recording": "node",
  "sid": "46f82471-b834-41a9-a50b-6ee326a692af",
  "size": "141:22",
  "time": "2023-09-21T15:50:32.784Z",
  "uid": "5827ab06-f793-4fae-b680-b067aa02f1f0",
  "user": "alice"
}

[public-teleport-github-review-bot]

@gabrielcorado See the table below for backport results.

Branch	Result
branch/v12	Create PR
branch/v13	Create PR
branch/v14	Create PR