Service Ping instrumentation classes - add customization of timestamp column in database metric

What does this MR do and why?

This commit enhance database instrumentation DSL with timestamp_column keyword which enables users to define custom timestamp column for their metrics. By default timestamp column is created_at.

Resolves #364345 (closed)

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How to set up and validate locally

There is no metric that uses new DSL keyword, so in order to validate MR changes one needs to modify existing one eg:

diff --git a/lib/gitlab/usage/metrics/instrumentations/count_issues_metric.rb b/lib/gitlab/usage/metrics/instrumentations/count_issues_metric.rb
index 07dfba70d92..44ea93b97b2 100644
--- a/lib/gitlab/usage/metrics/instrumentations/count_issues_metric.rb
+++ b/lib/gitlab/usage/metrics/instrumentations/count_issues_metric.rb
@@ -12,7 +12,8 @@ class CountIssuesMetric < DatabaseMetric

           relation { Issue }

-          cache_start_and_finish_as :issue
+          # cache_start_and_finish_as :issue
+          timestamp_column :last_edited_at
         end
       end
     end

Than check how many records matches 28d time constraint

> Issue.where(last_edited_at: 30.days.ago..2.days.ago).count
=> 0

If non then update one of them to have some data

Issue.first.update(last_edited_at: 5.days.ago)

Than finally use metric instrumentation class to collect the same data

Gitlab::Usage::Metrics::Instrumentations::CountIssuesMetric.new(time_frame: '28d').value
=> 1

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.