Refactor code to substitute variables in Prometheus queries

What does this MR do?

Refactor the code for substituting variables in Prometheus queries.

Changes this MR contains:

1. Create service for substituting variables using the Ruby interpolation format (string_%{variable}_string)
   app/services/template_engines/ruby_service.rb.

2. Create service for substituting variables in the params of the Prometheus Proxy API. This is in preparation for adding support for the Liquid template format in !20793 (merged).
   app/services/prometheus/proxy_variable_substitution_service.rb

Issue: #35242 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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