feat: add optional extra configs for agent and cluster receiver

[chenlujjj]

Description:

Now the chart has defined some configs in the templates/config folder, and allow users to override them in the config part of the values.yaml file. For most use cases, it works perfect.

However, in our company there are more than 100 k8s clusters and the collector configuration differs here and there across these clusters. At the beginning we maintain the specific configs in each cluster's collector values.yaml file, but it quickly becomes a heavy burden. We need templating functionality for these extra configs, but helm doesn't support templating for values at the moment. So we come up with an idea: allow the user provide self-defined configmap as the collector configuration, which will be appended to the running command of the collector container.

For example, the command is /otelcol --config=/conf/relay.yaml now. If there exists extra configs, it will be /otelcol --config=/conf/relay.yaml --config=/conf-extra/relay.yaml

Link to Splunk idea: <Link to Splunk idea, see https://ideas.splunk.com>

Testing:

Documentation:

[jinja2]

Hi @chenlujjj. Thank you for opening this PR. It would be best to file a Splunk idea to add this feature to our helm chart. Thanks!

[chenlujjj]

Hi @jinja2 ,I've file a idea here: https://ideas.splunk.com/ideas/EID-I-2345