prometheus: promtool check config complains about missing region in EC2 SD config

Use case. Why is this important?

I want to validate my prometheus configuration file

Bug Report

What did you do?

I run promtool check config against my prometheus.yml and got an error saying:

Checking packer/files/prometheus/config/prometheus.yml
  FAILED: parsing YAML file packer/files/prometheus/config/prometheus.yml: EC2 SD configuration requires a region

What did you expect to see?

I expected to see no errors as not having a region in the EC2 SD configuration is supported. Per the documentation: https://prometheus.io/docs/prometheus/latest/configuration/configuration/#ec2_sd_config

# The AWS region. If blank, the region from the instance metadata is used.

Environment

• System information:

  Linux 4.15.0-65-generic x86_64

• Prometheus version:

prometheus, version 2.12.0 (branch: HEAD, revision: 43acd0e2e93f9f70c49b2267efa0124f1e759e86)
  build user:       root@7a9dbdbe0cc7
  build date:       20190818-13:53:16
  go version:       go1.12.8

• Prometheus configuration file:

global:
  scrape_interval: 10s

rule_files:
  - 'alerts/*.yml'

alerting:
  alertmanagers:
  - static_configs:
    - targets:
      - localhost:9093

scrape_configs:
  - job_name: 'prometheus'
    ec2_sd_configs:
      - port: 9090
    relabel_configs:
      - source_labels: [__meta_ec2_tag_Name]
        regex: prometheus
        action: keep
      - source_labels: [__meta_ec2_tag_Name]
        target_label: instance
      - source_labels: [__meta_ec2_tag_Environment]
        target_label: environment