fix(prometheusrule): set nindent to 2

[wallchristopher]

Fixes #1428

If default rules are enabled and passing in additional groups, there appears to be whitespace mismatch causing helm template to error out. Giving Error: YAML parse error on opentelemetry-collector/templates/prometheusrule.yaml: error converting YAML to JSON: yaml: line 24: did not find expected key

Setting either the nindent to 2 or moving all defualt rules by two spaces fixes the issue.

Template using test values

spec:                                                                                                                                                                                                                                                  
  groups:                                                                                                                                                                                                                                              
    - name: additionalCollectorRules                                                                                                                                                                                                                   
      rules:                                                                                                               
      - alert: ExporterQueueFull                    
        annotations:                                                                                                       
          description: Queue of exporter whatever is more than 90% full
        expr: 100 * sum by (exporter) (otelcol_exporter_queue_size) / sum by (exporter)                                    
          (otelcol_exporter_queue_capacity) > 90                                                                           
        for: 2m                                                                                                            
        labels:                                                                                                            
          severity: critical                     
  - name: collectorRules
    rules:                                             
    - alert: ReceiverDroppedSpans               
      expr: rate(otelcol_receiver_refused_spans[5m]) > 0
      for: 2m                                                                                                              
      labels:                                                                                                              
        severity: critical                
      annotations:                                                                                                         
        description: 'The {{ $labels.receiver }} receiver is dropping spans at a rate of {{ humanize $value }} per second ' 
        runbook_url: 'https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/monitoring.md#receive-failures'

Test Values

serviceMonitor:
  enabled: true

prometheusRule:
  enabled: true
  groups:
  - name: additionalCollectorRules
    rules:
    - alert: ExporterQueueFull
      expr: 100 * sum by (exporter) (otelcol_exporter_queue_size) / sum by (exporter) (otelcol_exporter_queue_capacity) > 90
      for: 2m
      labels:
        severity: critical
      annotations:
        description: Queue of exporter whatever is more than 90% full
  defaultRules:
    enabled: true