Add default_labels config option

[clupprich]

Adds a default_labels config option to Cloud Trace. The labels defined there will be added to every root span.

For example, this config:

Rails.application.configure do |config|
  config.google_cloud.trace.default_labels = {
      '/component' => 'test-component'
    }
end

will result in the /component label being added:

[dazuma]

Can we use ||= so that the direct client config takes precedence over Rails config (for consistency with all the other configs here—note that they all use ||= or equivalent).

[clupprich]

||= only works on nil, but the default here is an empty Hash.

We could introduce a safeguard into the set_default_labels method that verifies we're not dealing with nil. I find the current solution more elegant, however, cause it pushes that logic to the boundary.

Let me know what you think!