certificate_file_expiry: optional logarithmic view for long lived certs

logarithmic was removed in d885a314ad
but still has it's benefits for openvpn CA and CRL certs. when enabling
logarithmic we keep the SI units now instead of rrdtools default
scientific notation.

plugins/ssl/certificate_file_expiry

@@ -13,6 +13,7 @@ For openvpn ca.crt and crl.pem
  [certificate_file_expiry]
  user root
  env.CERTS crl:/etc/openvpn/easy-rsa/keys/crl.pem x509:/etc/openvpn/easy-rsa/keys/ca.crt
+ env.LOGARITHMIC yes
 
 For letsencrypt certificates
 
@@ -26,6 +27,8 @@ Warning and Critical levels can also be configured with env variables like this:
  ...
  # warn when certificate will be invalid within 5 days
  env.warning 5:
+ # for this certificate warn us 10 days before because it takes longer to renew
+ env._etc_letsencrypt_live_example_com_cert_pem_warning 10:
  # critical when certificate will be invalid within 1 day
  env.critical 1:
 
@@ -35,7 +38,11 @@ File patterns can be a single file (e.g. /etc/openvpn/easy-rsa/keys/crl.pem) or
 (e.g. /etc/letsencrypt/live/*/cert.pem).
 
 env.warning and env.critical are configurable values for the warning and critical levels according to
-http://munin-monitoring.org/wiki/fieldname.warning and http://munin-monitoring.org/wiki/fieldname.critical
+http://guide.munin-monitoring.org/en/latest/tutorial/alert.html?highlight=warning#syntax-of-warning-and-critical
+
+env.LOGARITHMIC "yes" enables the logarithmic display of values which is useful if some of your certs are relatively
+long lived in respect to the warning level. e.g. a ca.crt that is valid for 10 years together with a crl.pem that is
+valid for only a few months combined with warning levels of 5 days. default is "yes" to disable set it to "no".
 
 =head1 Dependencies
 
@@ -53,16 +60,19 @@ GPLv2
 
 . "$MUNIN_LIBDIR/plugins/plugin.sh"
 
+LOGARITHMIC=${LOGARITHMIC:-yes}
+
 if [ "$1" = "config" ] ; then
   echo "graph_title Certificate validity"
-  echo "graph_args --base 1000"
+  if [ "$LOGARITHMIC" = "yes" ] ; then
+    graph_args="--logarithmic --units=si"
+  fi
+  echo "graph_args --base 1000 $graph_args"
   echo "graph_vlabel days"
   echo "graph_category security"
 fi
 
 now=$(date +%s)
-warning=${warning:-5:}
-critical=${critical:-1:}
 for cert in ${CERTS}; do
   cert_type=${cert%:*}
   cert_pattern=${cert#*:}