Add tolerant version regex parsing to handle PostgreSQL 10.

Closes prometheus-community#112.

Author: wrouesnel

postgres_exporter.go

@@ -102,15 +102,15 @@ func (cu *ColumnUsage) UnmarshalYAML(unmarshal func(interface{}) error) error {
 }
 
 // Regex used to get the "short-version" from the postgres version field.
-var versionRegex = regexp.MustCompile(`^\w+ (\d+\.\d+\.\d+)`)
+var versionRegex = regexp.MustCompile(`^\w+ ((\d+)(\.\d+)?(\.\d+)?)\s`)
 var lowestSupportedVersion = semver.MustParse("9.1.0")
 
 // Parses the version of postgres into the short version string we can use to
 // match behaviors.
 func parseVersion(versionString string) (semver.Version, error) {
 submatches := versionRegex.FindStringSubmatch(versionString)
 if len(submatches) > 1 {
-return semver.Make(submatches[1])
+return semver.ParseTolerant(submatches[1])
 }
 return semver.Version{},
 errors.New(fmt.Sprintln("Could not find a postgres version in string:", versionString))