node_exporter compatible textfile generator for certificate expiry

certificate-metrics-textfile.py

#!/usr/bin/python3

from pathlib import Path
from cryptography import x509
import time

textfile = open('/path/to/metrics', 'w')

textfile.write('# HELP ssl_certificate_expiry Unix timestamp of certificate expiry\n')
textfile.write('# TYPE ssl_certificate_expiry gauge\n')

certs_path = Path('/path/to/certificates')
for certificate_path in certs_path.glob('*.crt'):
    if certificate_path.match('*.issuer.crt'):
        continue
    certificate_bytes = open(certificate_path, 'rb').read()
    certificate = x509.load_pem_x509_certificate(certificate_bytes)
    textfile.write('ssl_certificate_expiry{path="' + str(certificate_path) + '"} ' + certificate.not_valid_after.strftime('%s') + '\n')