Return 200 if no hosts found

[meiji163]

Currently if freno finds no backend hosts to collect metrics from it returns 500. We find it preferable to fail open in this case and return 200. For example if hosts are unavailable from one "domain" we can still operate with the metrics from the other domains.

[Copilot]

Pull Request Overview

This PR modifies the throttler's behavior to return HTTP 200 status code when no hosts are found, rather than treating it as an error condition.

• Export the NoHostsError variable to make it publicly accessible for error checking
• Add specific handling for NoHostsError in the throttler check logic to return HTTP 200
• Add comprehensive test coverage for the throttler check functionality including the new no-hosts behavior

Reviewed Changes

Copilot reviewed 6 out of 30 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/base/throttle_metric.go	Export NoHostsError by capitalizing the variable name
pkg/throttle/check.go	Add error handling to return HTTP 200 when no hosts are found
pkg/throttle/throttler_test.go	Add comprehensive test suite for throttler check behavior
pkg/throttle/check_result.go	Reorder imports for consistency
pkg/mysql/mysql_inventory.go	Add blank line for import formatting
go.mod	Add testify dependency for testing

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