- Closes #60024

- Change PrometheusClient.new to accept a base url instead of an
  already created RestClient

- Use Gitlab::HTTP in PrometheusClient instead of creating RestClient
  in PrometheusService

- Move http_options from PrometheusService to
  PrometheusClient (follow_redirects: false)

- ensure that base urls don't have the trailing slash

- Created a `PrometheusClient#url` method that might not be strictly
  required

- Change rescued exceptions from RestClient::* to
  HTTParty::ResponseError where possible and StandardError for the
  rest

Prior this commit exceptions raised during a HTTP request
weren't caught by the Sentry client and were passed to the user.

In addition the Sentry client tried to catch a non-existent error
`Sentry::Client::SentryError`.

Now, the Sentry client catches all possible errors coming from
a HTTP request.

- Do not raise error when there are missing non-essential keys in sentry
api response.
- Add specs for to check for missing keys behavior.

Return the exception as an error.