Add service ping for exact code search

What does this MR do and why?

Add a metric class ZoektSearchEnabledMetric to track if a self managed customer has zoekt enabled in its application setting.

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Before	After

How to set up and validate locally

When setting zoekt_search_enabled is true

Run the following command in rails console

ApplicationSetting.last.update(zoekt_settings: {"zoekt_search_enabled"=>true, "zoekt_indexing_paused"=>false, "zoekt_indexing_enabled"=>true})

Now run the following script on terminal

bin/rake gitlab:usage_data:dump_sql_in_json | jq '.settings'

Notice that zoekt_search_enabled is true in the JSON

When setting zoekt_search_enabled is false

Run the following command in rails console

ApplicationSetting.last.update(zoekt_settings: {"zoekt_search_enabled"=>false, "zoekt_indexing_paused"=>false, "zoekt_indexing_enabled"=>true})

Now run the following script on terminal

bin/rake gitlab:usage_data:dump_sql_in_json | jq '.settings'

Notice that zoekt_search_enabled is false in the JSON

Related to #461686 (closed)