Add a small http hook to OpenTelemetry::Exporter::OTLP::Metrics::MetricsExporter

OpenTelemetry::Exporter::OTLP::Metrics::MetricsExporter is hard to extend for small HTTP customizations.

For example, I want to sign outgoing requests, but currently I need to reimplement the whole send_bytes method.

It would be great to have a small hook that runs after the request is built and before it is sent, e.g.:

def customize_request(request)
  request
end

Then custom exporters could do:

class SignedMetricsExporter < OpenTelemetry::Exporter::OTLP::Metrics::MetricsExporter
  private

  def customize_request(request)
    request['Authorization'] = signer.sign(request)
    request
  end
end

Lmk if you'd be open to such a change then I'd try to create a PR for it. Feel free to close the issue otherwise :)

Start with OpenTelemetry::Exporter::OTLP::Metrics::MetricsExporter and trace its send_bytes method to find where the request is built and sent. Add the requested customization hook at that boundary, preserving the request flow and allowing subclasses to modify it. Done means a custom exporter can add an Authorization header without reimplementing send_bytes, with focused coverage for the hook behavior.