GithubImporter: Fix import duration metric

Since `project.full_path` is user provided information, this means that
the cardinality of this metric is not bound and we can't initialize the
metric properly. Because the metric isn't initialized and the project is
included we can never track the rate past 1 for each counter in the
histogram: A histogram is a series of counters, in total there are
`number of buckets * number of label combinations` counters that
Prometheus uses to calculate the quantiles. For that, we're using a
[rate](https://prometheus.io/docs/prometheus/latest/querying/functions/#rate),
which is the increase of a counter in seconds.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/341958

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74610

lib/gitlab/import/metrics.rb

@@ -69,11 +69,7 @@ module Gitlab
       end
 
       def observe_histogram
-        if project.github_import?
-          duration_histogram.observe({ project: project.full_path }, duration)
-        else
-          duration_histogram.observe({ importer: importer }, duration)
-        end
+        duration_histogram.observe({ importer: importer }, duration)
       end
 
       def track_finish_metric

spec/lib/gitlab/import/metrics_spec.rb

@@ -94,20 +94,6 @@ RSpec.describe Gitlab::Import::Metrics, :aggregate_failures do
         expect(histogram).to have_received(:observe).with({ importer: :test_importer }, anything)
       end
     end
-
-    context 'when project is a github import' do
-      before do
-        project.import_type = 'github'
-      end
-
-      it 'emits importer metrics' do
-        expect(subject).to receive(:track_usage_event).with(:github_import_project_success, project.id)
-
-        subject.track_finished_import
-
-        expect(histogram).to have_received(:observe).with({ project: project.full_path }, anything)
-      end
-    end
   end
 
   describe '#issues_counter' do