Emit RegistrationLatencySeconds metric from role::github_runner

[akuzminsky]

Context

terraform-aws-actions-runner is adding a dashboard and alarms for the
runner fleet (infrahouse/terraform-aws-actions-runner#93). One of the
highest-value missing signals is time-to-register — how long it takes
from an EC2 instance booting to that instance being available as a
registered GitHub Actions runner. That covers apt, Puppet, and
./config.sh in a single operator-facing number.

There is no CloudWatch metric for this today. This issue tracks adding it
on the instance side via Puppet.

Proposed change

In profile::github_runner::register, right after ./config.sh succeeds,
emit:

aws cloudwatch put-metric-data \
  --region "$AWS_REGION" \
  --namespace GitHubRunners \
  --metric-name RegistrationLatencySeconds \
  --dimensions asg_name=\$ASG_NAME \
  --unit Seconds \
  --value \"\$(awk '{print \$1}' /proc/uptime)\"

• Measurement: instance uptime (/proc/uptime) at the moment
  registration completes. Captures apt + Puppet + ./config.sh in one
  operator-facing number.
• Emission point: the instance itself, after ./config.sh.
• Permissions: instance profile already provides general CloudWatch
  access; verify `cloudwatch:PutMetricData` is allowed before shipping.
  If not, add it to the role used by `role::github_runner`.
• Namespace / dimensions: match the existing `GitHubRunners`
  namespace / `asg_name` dimension already used by the module's
  `record_metric` Lambda (BusyRunners / IdleRunners).

Why

• Accurate — the moment we know the runner is live.
• Simple — one `aws cloudwatch put-metric-data` call.
• No new AWS infra (no Lambda, no EventBridge, no DynamoDB).

Follow-up (in terraform-aws-actions-runner)

Once this lands, a follow-up PR in the runner module will:

1. Surface `RegistrationLatencySeconds` as a dashboard widget (p50 / p95).
2. Add an alarm, e.g. p95 > 20 min sustained, so slow-boot regressions page.

The module's dashboard already includes a placeholder widget for this
metric in PR #93; it will show "No data" until this issue ships.