renamed GCP projects in providerFlags

[ohyesgocool]

Rename gcp.bucket-projects to gcp.projects for clarity

This merge request improves the user experience when configuring GCP projects by introducing a more intuitive flag name. The existing gcp.bucket-projects flag is currently used for all GCP metrics (not just bucket metrics), which is confusing and counterintuitive.

Changes made:

1. Added a new gcp.projects flag in exporter.go that serves the same purpose as the legacy flag
2. Updated the developer guide to reflect the new recommended flag usage

[ohyesgocool]

@Pokom should we also add a log warn for the existing users ? Something like - The flag 'gcp.bucket-projects' is deprecated, instead use gcp.projects 😃

[Pokom]

@nikimanoledaki when you have a moment, can you take a peak at this? I believe we've discussed this internally a few times, but might be a good opportunity for @leonorfmartins to review as well.

[cindy]

@Pokom should we also add a log warn for the existing users ? Something like - The flag 'gcp.bucket-projects' is deprecated, instead use gcp.projects 😃

Hello there! Sorry this has taken so long to get back to you. I wonder if it would be a good idea to deprecate this flag and add the new one and also print out a warning that the old one is deprecated and a warning like "'gcp.bucket-projects' is deprecated and will be removed in a future version. Use '--gcp.projects' instead."