fix: always re-add config values provided with --config after internal config values

[obs-gh-mattcotter]

Description

After switching to the otelcol's internal --config flag, the bundled config files provided by the observe-agent now are processed after any configuration provided with --config. This makes it so the bundled config values cannot be overwritten via the --config flag (only via the otel_config_overrides option). This fix makes the flag take precedence again.

Ex:

$ cat ./hc_test.yaml 
extensions:
  health_check:
    endpoint: 0.0.0.0:13137

Current 2.0.0:

$ observe-agent start --config ./hc_test.yaml 
# ...
2025-02-10T11:10:54.862-0600	info	healthcheckextension/healthcheckextension.go:32	Starting health_check extension	{"kind": "extension", "name": "health_check", "config": {"Endpoint":"localhost:13133","TLSSetting":null,"CORS":null,"Auth":null,"MaxRequestBodySize":0,"IncludeMetadata":false,"ResponseHeaders":null,"CompressionAlgorithms":null,"ReadTimeout":0,"ReadHeaderTimeout":0,"WriteTimeout":0,"IdleTimeout":0,"Path":"/status","ResponseBody":null,"CheckCollectorPipeline":{"Enabled":false,"Interval":"5m","ExporterFailureThreshold":5}}}

^ note the "Endpoint":"localhost:13133"

After this fix:

$ ./observe-agent start --config ./hc_test.yaml 
# ...
2025-02-10T11:11:41.227-0600	info	healthcheckextension/healthcheckextension.go:32	Starting health_check extension	{"kind": "extension", "name": "health_check", "config": {"Endpoint":"0.0.0.0:13137","TLSSetting":null,"CORS":null,"Auth":null,"MaxRequestBodySize":0,"IncludeMetadata":false,"ResponseHeaders":null,"CompressionAlgorithms":null,"ReadTimeout":0,"ReadHeaderTimeout":0,"WriteTimeout":0,"IdleTimeout":0,"Path":"/status","ResponseBody":null,"CheckCollectorPipeline":{"Enabled":false,"Interval":"5m","ExporterFailureThreshold":5}}}

^ note the "Endpoint":"0.0.0.0:13137"

Checklist

• Created tests which fail without the change (if possible)
• Extended the README / documentation, if necessary

[obs-gh-alexlew]

why are these guys changing?

[obs-gh-mattcotter]

Terraform couldn't find the old AMIs. The published images change from time to time, but I don't know why AWS does that.

[obs-gh-alexlew]

this part is sort of confusing/seems unfortunate to me. are the first 2 arguments special in some way?

[obs-gh-mattcotter]

This is referring to the string; two characters only means [] which means no args. This skips the string split when we don't need to do it basically.

[obs-gh-mattcotter]

Oh and one more weird thing: calling Split on empty string does not return empty array but instead returns array of empty string.
https://go.dev/play/p/wZ3He29DP37

So this saves from filtering out empty string when looping over the configs below.