Possibility to set Loki tenant id from resource attributes

[thmshmm]

Describe the issue you're reporting

Currently the Loki exporter supports setting a dynamic tenant id based on a resource attribute using the tenant field. This field is deprecated and will be removed in v0.61.

The suggested alternative to use the "headers_setter" does not support resource attributes.

As proposed by @jpkrohling on Slack, something like a "loki.attributes.tenant" label could be introduced using a processor which can be picked up by the exporter to be set as tenant in the header.

[jpkrohling]

@kovrus volunteered to work on this one.

[jpkrohling]

Issue #12873 mentions the tenant header, and I think I like the name I proposed there better than what I added as a comment via Slack: loki.tenant. We could look it up first in the resource attributes, falling back to record attributes.

[kovrus]

It should be closed after merging #15285.