Provide ways to include new attributes to span

[ArthurSens]

For preview environments, for example, we used to add a new attribute preview.domain so we can use it to filter traces from different preview environments.

How it was done with jsonnet:

observability/components/open-telemetry-collector/open-telemetry-collector.libsonnet

Lines 32 to 42 in 6fb43f5

	local processorsConfig = if withPreviewEnvironment then
	|||
	processors:
	attributes:
	actions:
	- key: 'preview.domain'
	value: %(domain)s
	action: insert
	||| % {
	domain: config.previewEnvironment.domain,
	} else '',

Our installer should be able to add attributes as well