Prevent usage of unkeyed struct literals for all Settings and types with exported members

[bogdandrutu]

We need to be able to expand our Settings structs, see:

type TelemetrySettings struct {
	Logger *zap.Logger
	TracerProvider trace.TracerProvider
	MeterProvider metric.MeterProvider
	Resource pcommon.Resource
}

// Compiles fine, which means adding a new member will cause this to fail.
ts := component.TelemetrySettings{zap.NewNop(), nooptrace.NewTracerProvider(), noopmetric.NewMeterProvider(), pcommon.NewResource()}

Proposal use a blank field member (_ struct{}) to disallow unkeyed literals initialization.

type TelemetrySettings struct {
	Logger *zap.Logger
	TracerProvider trace.TracerProvider
	MeterProvider metric.MeterProvider
	Resource pcommon.Resource
	_ struct{} // Prevents unkeyed literals
}

// Compilation error without the last struct.
ts := component.TelemetrySettings{zap.NewNop(), nooptrace.NewTracerProvider(), noopmetric.NewMeterProvider(), pcommon.NewResource()}

// Compilation error because cannot initialize blank member.
ts := component.TelemetrySettings{zap.NewNop(), nooptrace.NewTracerProvider(), noopmetric.NewMeterProvider(), pcommon.NewResource(), struct{}{}}

In the main versioning guarantees we reserve the right to add new fields, see https://github.com/open-telemetry/opentelemetry-collector/blob/main/VERSIONING.md#general-go-api-considerations so even a module is stable we can still do this change.

[mx-psi]

I think it is useful to add this to all structs that don't have a lot of fields

[atoulme]

Here is a list:

• connector/forwardconnector struct "Config" does not prevent unkeyed literal initialization
• exporter/debugexporter struct "Config" does not prevent unkeyed literal initialization
• exporter/otlpexporter struct "Config" does not prevent unkeyed literal initialization
• exporter/otlphttpexporter struct "EncodingType" does not prevent unkeyed literal initialization
• extension/memorylimiterextension struct "Config" does not prevent unkeyed literal initialization
• extension/zpagesextension struct "Config" does not prevent unkeyed literal initialization
• processor/batchprocessor struct "Config" does not prevent unkeyed literal initialization
• processor/memorylimiterprocessor struct "Config" does not prevent unkeyed literal initialization
• receiver/otlpreceiver struct "Protocols" does not prevent unkeyed literal initialization
• scraper struct "Settings" does not prevent unkeyed literal initialization
• scraper/scraperhelper struct "ControllerOption" does not prevent unkeyed literal initialization
• service struct "Settings" does not prevent unkeyed literal initialization