Make Sidekiq exporter logs conform to Sidekiq logs

Sidekiq logs generates `exception.message`, `exception.class`, and
`exception.backtrace`, while the Sidekiq exporter uses the string
`exception` for the listen error. To avoid conflicting schema types in
Elasticsearch, change `exception` to `exception.message`.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/215381

lib/gitlab/metrics/exporter/sidekiq_exporter.rb

@@ -32,7 +32,7 @@ module Gitlab
           Sidekiq.logger.error(
             class: self.class.to_s,
             message: 'Cannot start sidekiq_exporter',
-            exception: e.message
+            'exception.message' => e.message
           )
 
           false

spec/lib/gitlab/metrics/exporter/sidekiq_exporter_spec.rb

@@ -53,7 +53,7 @@ describe Gitlab::Metrics::Exporter::SidekiqExporter do
         .with(
           class: described_class.to_s,
           message: 'Cannot start sidekiq_exporter',
-          exception: anything)
+          'exception.message' => anything)
 
       exporter.start
     end