Configure header on Prometheus Remote Write sink

[Shuricky]

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Problem

Currently on the prometheus remote write sink there is no way to stamp custom HTTP headers. The only header that you can stamp is X-Scope-OrgID using the tenant_id config.

For comparison, the HTTP sink does allow configuring custom headers.

Proposal

Create a new field on the prometheus remote write sink called headers that will contain one or more header name to header value mappings. For example:

sinks:
  my_sink_id:
    type: prometheus_remote_write
    inputs:
      - my-source-or-transform-id
    tenant_id: my-domain
    header:
      "header-name": header-value