Add Prometheus metrics relabel configuration for exporters

Tested succesfully with the following test cases:
            source_labels: instance
            source_labels: instance,__name__,test
            source_labels: [instance,__name__,test]
            source_labels:
              - instance
              - __name__
              - test

Change-Id: I9750bb0a4204335dd202cc4020445e850824c497
Depends-On: I2f85fe2f9cc67d2d03d5e8dad5ac0b74e82b0fcc

prometheus/files/prometheus.yml

@@ -47,9 +47,15 @@ rule_files:
     {%- if static_target[job_name]['endpoint'] is not defined %}
       {%- do static_target[job_name].update({'endpoint': []}) %}
     {%- endif %}
+    {%- if static_target[job_name]['metric_relabel'] is not defined %}
+      {%- do static_target[job_name].update({'metric_relabel': []}) %}
+    {%- endif %}
     {%- for target in job.get('endpoint', []) %}
       {%- do static_target[job_name]['endpoint'].append(target) %}
     {%- endfor %}
+    {%- for label in job.get('metric_relabel', []) %}
+      {%- do static_target[job_name]['metric_relabel'].append(label) %}
+    {%- endfor %}
   {%- endfor %}
 {%- endfor %}
 
@@ -71,6 +77,22 @@ scrape_configs:
     {%- endif %}
     static_configs:
     - targets: [{{ nodes | join(',') }}]
+    {%- if job.get('metric_relabel') %}
+    metric_relabel_configs:
+      {%- for label in job.get('metric_relabel', []) %}
+        {%- if label.source_labels is string %}
+          {%- set label_list = label.source_labels.split(',') %}
+        {%- elif label.source_labels is list %}
+          {%- set label_list = label.source_labels %}
+        {%- endif %}
+        {%- if label_list is defined %}
+      - source_labels: {{ label_list }}
+        regex: "{{ label.regex }}"
+        target_label: "{{ label.target_label }}"
+        replacement: {{ label.replacement }}
+        {%- endif %}
+      {%- endfor %}
+    {%- endif %}
   {%- endif %}
 {%- endfor %}