*: move to protobuf v2 api + vtprotobuf

[GiedriusS]

Builds on top of our previous work. I think we could merge this on green.

Pros:

• We are getting rid of a huge unmaintained dependency;
• It unblocks enabling pooling for SeriesResponses.

Implementation notes:

• We are now downloading protoc compiler with needed includes from the Vitess project; in the future let's do this package ourselves? This is needed because IIRC upstream doesn't provide a trimmed zip with only the compiler + protobuf includes
• All generated types became pointers because upstream protobuf v2 API doesn't support the previously used extension
• ~~I think this will force all of our downstream users i.e. Cortex/Mimir to upgrade to the protobuf V2 API~~ now that Cortex is in included in our repo, I have made the respective changes + regenerated protobuf files using the new compiler

TODO in future PRs:

• Bring back ZLabels - it's a bit trickier now because we will have to use sed to delete methods;

[stale[bot]]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[matej-g]

@GiedriusS what's missing here? (Except resolving that terrible conflict list :slightly_smiling_face:) Any way to support here?

[stale[bot]]

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