Consider separating configuration files

[james-bebbington]

Viper, the library used to load configuration in the Collector, supports loading from and merging multiple config files. Note the following caveats:

• Arrays are overwritten instead of being merged
• There is no mechanism to "include" config files from within another config file

We could change the startup code to load all *.yaml files in the same directory as the executable (or based on some provided regex flag) and change the configuration to be logically separated, e.g.:

<config.yaml>

# shared components listed here
processors:
  resourcedetection:
    ...

<config_agent.yaml>

pipelines:
  metrics/agent:
    receivers: [ prometheus/agent ]
    processors: [ metricstransform/agent, resourcedetection ]
    exporters: [ ... ]

receivers:
  prometheus/agent:
    # scrape self reported agent metrics

processors:
  metricstransform/agent:
    ...

<config_iis.yaml>

pipelines:
  metrics/iis:
    receivers: [ windowsperfcounters/iis ]
    processors: [ metricstransform/iis, resourcedetection ]
    exporters: [ ... ]

receivers:
  windowsperfcounters/iis:
    ...

processors:
  metricstransform/iis:
    ...

<config_sqlserver.yaml>

pipelines:
  metrics/sqlserver:
    receivers: [ windowsperfcounters/sqlserver ]
    processors: [ metricstransform/sqlserver, resourcedetection ]
    exporters: [ ... ]

receivers:
  windowsperfcounters/sqlserver:
    ...

processors:
  metricstransform/sqlserver:
    ...

---

In addition, it would be useful to support a way of loading constants into environment variables so that values don't need to be repeated across config files, e.g.:

<config_env.yaml>
CUSTOM_METRIC_PREFIX: custom.googleapis.com/

<config_*.yaml>

exporters:
  stackdriver/application1:
    metric:
      prefix: $CUSTOM_METRIC_PREFIX

  stackdriver/application2:
    metric:
      prefix: $CUSTOM_METRIC_PREFIX