Update to otel-go v1.0.0-RC1

[0xc0d]

Since OpenTelemetry has been updated to v1.0.0-RC1 I am sending this PR.

Changes are:

• exporter now satisfies otel Exporter interface which needs a slice of ReadOnlySpan

ExportSpans(context.Context, []*SpanSnapshot) error
ExportSpans(context.Context, []ReadOnlySpan) error 

• WithDefaultTraceAttributes function and DefaultTraceAttributes are removed since it is not possible to add attributes to ReadOnlySpan
• injectLabelsFromResources is removed since you can't inject anything to a ReadOnlySpan. Instead, attributeWithLabelsFromResources returns all attributes with labels from resources.
• Tests for injectLabelsFromResources are removed since mocking ReadOnlySpan could lead to a problem in future. Although it might be an option to change attributeWithLabelsFromResources's signature to get an interface like this:

type AttributeGetter interface {
    Attributes() []attribute.KeyValue
    Resource() *resource.Resource
}

With this we can mock an interface to be able to test this function.

[punya]

Thanks for contributing this @0xc0d !