Granular control for metric collection

We see an issue where a BMC is reporting 42+ ethernet
interfaces and not returning any data for them. This is likely
a bug in the BMC FW.

This patch allows skipping the collection of certain groups
of metrics to work around the bug. It adds an optional
configuration section which can be used to control what is
collected.

Author: dougszumski

README.md

@@ -16,6 +16,11 @@ groups:
   group1:
     username: group1_user
     password: group1_pass
+  flakey:
+    username: "Administrator"
+    password: "Password"
+    disabled_metrics:
+      - "ethernet_interfaces"
 ```
 Note that the ```default``` entry is useful as it avoids an error
 condition that is discussed in [this issue][2].
@@ -60,6 +65,35 @@ curl '127.0.0.1:9123/redfish?target=10.10.12.23&collectlogs=true'
 
 The `collectlogs` query parameter can be included in Prometheus config.
 
+## Disabling collection of specific groups of metrics
+
+Sometimes it isn't possible to gather all metrics from a BMC because
+it doesn't conform to the Redfish standard due to a bug. Or perhaps
+you don't need all the metrics. In either case, basic support exists
+for skipping specific metric groups at the `default`, `group` or
+`host` level:
+
+```yaml
+hosts:
+  10.36.48.24:
+    username: admin
+    password: pass
+    disabled_metrics:
+      - "ethernet_interfaces"
+  default:
+    username: admin
+    password: pass
+    disabled_metrics:
+      - "memory"
+      - "processor"
+      - "storage"
+      - "pcie_devices"
+      - "network_interfaces"
+      - "ethernet_interfaces"
+      - "simple_storage"
+      - "pcie_functions"
+```
+
 ## Building
 
 To build the redfish_exporter executable run the command:

collector/redfish_collector.go

@@ -40,15 +40,15 @@ type RedfishCollector struct {
 }
 
 // NewRedfishCollector return RedfishCollector
-func NewRedfishCollector(host string, username string, password string, collectLogs bool, logger *log.Entry) *RedfishCollector {
+func NewRedfishCollector(host string, username string, password string, collectLogs bool, disabledMetrics []string, logger *log.Entry) *RedfishCollector {
 	var collectors map[string]prometheus.Collector
 	collectorLogCtx := logger
 	redfishClient, err := newRedfishClient(host, username, password)
 	if err != nil {
 		collectorLogCtx.WithError(err).Error("error creating redfish client")
 	} else {
 		chassisCollector := NewChassisCollector(redfishClient, collectLogs, collectorLogCtx)
-		systemCollector := NewSystemCollector(redfishClient, collectLogs, collectorLogCtx)
+		systemCollector := NewSystemCollector(redfishClient, collectLogs, disabledMetrics, collectorLogCtx)
 		managerCollector := NewManagerCollector(redfishClient, collectLogs, collectorLogCtx)
 
 		collectors = map[string]prometheus.Collector{"chassis": chassisCollector, "system": systemCollector, "manager": managerCollector}