sdk/log: Add filtering Processor example

[pellared]

Towards https://github.com/open-telemetry/opentelemetry-go/issues/5065

[pellared]

@MrAlias, do you think it makes sense to add MinSeverityProcessor as an example? I feel uncertain as I would personally would prefer to have it as part of the SDK. However, I am not sure when it happens and this looks like a good example before we can add it.

The alternative, I consider is something like the following:

func ExampleProcessor_filtering() {
    var processor logsdk.Processor = logsdk.NewBatchProcessor(nil)

    processor = &ContextIgnoringProcessor{processor}

    _ = logsdk.NewLoggerProvider(
        logsdk.WithProcessor(processor),
    )
}

type key struct{}

var igoreLogsKey key

// WithIgnoreLogs returns a context which is used by [ContextIgnoringProcessor]
// to filter out logs so that one does not want to process.
func WithIgnoreLogs(ctx context.Context) context.Context {
    return context.WithValue(ctx, igoreLogsKey, true)
}

func ignoreLogs(ctx context.Context) bool {
    _, ok := ctx.Value(igoreLogsKey).(bool)
    return ok
}

// ContextIgnoringProcessor filters out logs when a context deriving from
// [WithIgnoreLogs] is passed to its methods.
type ContextIgnoringProcessor struct {
    logsdk.Processor
}

func (p *ContextIgnoringProcessor) OnEmit(ctx context.Context, record logsdk.Record) error {
    if ignoreLogs(ctx) {
        return nil
    }
    return p.Processor.OnEmit(ctx, record)
}

func (p *ContextIgnoringProcessor) Enabled(ctx context.Context, record logsdk.Record) bool {
    return !ignoreLogs(ctx) && p.Enabled(ctx, record)
}

[MrAlias]

@MrAlias, do you think it makes sense to add MinSeverityProcessor as an example?

I was planning to add it to contrib. That way it can be used by users and when we want to stabilize it we can move it to the core repo.

[pellared]

@MrAlias, I plan to update the example to https://github.com/open-telemetry/opentelemetry-go/pull/5543#issuecomment-2194334650. Let me know if you have a better example proposal if you have any other suggestion.

[pellared]

Ready for another round of reviews after rework.