[otelcol] Make the ConfigProvider interface a struct

[evan-bradley]

Description

The ConfigProvider interface is not very useful since it isn't used by otelcol.Collector anymore since we moved to creating it through ConfigProviderSettings inside otelcol.Collector. Therefore, we should just make configProvider the concrete implementation and remove the interface. We could consider entirely removing ConfigProvider from the API, but it is used by some downstream distributions, and there is no other way to marshal YAML into otelcol.Config, so we would need to make additional changes to provide an adequate replacement.

We could also follow our deprecation process for this change, but in my opinion that will only cause more churn for downstream consumers since they will need to update their code multiple times throughout the process instead of just once.

[evan-bradley]

This is probably the biggest breaking change in this PR. If we return ConfigProvider instead of *ConfigProvider, it's likely that the change would go unnoticed by downstream users of this function. However, we typically return pointers for structs like this one, so not returning a point here would break that pattern.

The only field in this struct is a pointer, so from an efficiency standpoint I don't think it matters much whether this is a pointer or not.

[bogdandrutu]

I will ask a different question. Does this need to be public at all?

[evan-bradley]

My initial preference was to make everything in this file internal. However, if we do that, I see two issues:

1. There's no meaningful replacement for distro authors without forking this code, and I see quite a few usages. For example:

• https://github.com/observIQ/bindplane-otel-collector/blob/69053af3a2a2cbc23b994faf40865aaabff0b1c5/receiver/pluginreceiver/supplied_plugins_test.go#L109
• https://github.com/grafana/tempo/blob/60245c6c2e84d798a01d5af27b7325bdf16c1593/modules/distributor/receiver/shim.go#L214
• https://github.com/grafana/alloy/blob/60f43c5ad23f11e01c7e95829a4bd71baec29486/internal/converter/internal/otelcolconvert/otelcolconvert.go#L87
• https://github.com/SigNoz/signoz-otel-collector/blob/f10f88640de8cf52ce888b8c229949b2f111b9c2/pkg/collectorsimulator/collectorsimulator.go#L114
• https://github.com/xobserve/xobserve/blob/35c2745b913bae4a78dfa7ba212e24150190fc3b/otel-collector/pkg/config/factory.go#L39

2. There would be no way to unmarshal an otelcol.Config struct from the Collector's YAML, since all of the unmarshaling facilities are private. We should solve this separately.

My preference would be to make this smaller change for now, and reevaluate unexporting this at least after (2) is resolved.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.35%. Comparing base (fb06c99) to head (65c665c).
Report is 24 commits behind head on main.

Additional details and impacted files

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##             main   #12297   +/-   ##
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  Coverage   91.35%   91.35%           
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  Files         467      467           
  Lines       25761    25761           
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  Hits        23533    23533           
  Misses       1810     1810           
  Partials      418      418           

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