Make system firmware version optional in BMC info

There are systems that do not make this available, so don't make the
entire collector fail if only this metric can not be read. Instead, set
it to "N/A" if it cannot be determined.

This fixes prometheus-community#57.

README.md

@@ -294,6 +294,9 @@ version). Example:
 
     ipmi_bmc_info{firmware_revision="1.66",manufacturer_id="Dell Inc. (674)",system_firmware_version="2.6.1"} 1
 
+**Note:** some systems do not expose the system's firmware version, in which
+case it will be exported as `"N/A"`.
+
 ### Chassis Power State
 
 This metric is only provided if the `chassis` collector is enabled.

collector.go

@@ -592,8 +592,9 @@ func collectBmcInfo(ch chan<- prometheus.Metric, target ipmiTarget) (int, error)
 	}
 	systemFirmwareVersion, err := getBMCInfoSystemFirmwareVersion(output)
 	if err != nil {
-		log.Errorf("Failed to parse bmc-info data from %s: %s", targetName(target.host), err)
-		return 0, err
+		// This one is not always available.
+		log.Debugf("Failed to parse bmc-info data from %s: %s", targetName(target.host), err)
+		systemFirmwareVersion = "N/A"
 	}
 	ch <- prometheus.MustNewConstMetric(
 		bmcInfo,