etcdserver: improve Lease http path naming for gRPC gateway

[hexfusion]

This PR looks to normalize some of the lease api paths for the gRPC gateway. Using additional_bindings supported by grpc-gateway we can accept both old and new paths. Current plan is to support in 3.4 and deprecate all additional_bindings in 3.5. I will work on notifying existing clients of the change as well.

fixes #9430

/cc @akashbaid

[hexfusion]

I have no excuse to not add curl leases tests here ...

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@31de834). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #9450   +/-   ##
=========================================
  Coverage          ?   72.23%           
=========================================
  Files             ?      364           
  Lines             ?    30887           
  Branches          ?        0           
=========================================
  Hits              ?    22310           
  Misses            ?     6940           
  Partials          ?     1637

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 31de834...cd92d4a. Read the comment docs.

[skydoctor]

The parameters blocks such as this getting deleted in the auto-generated file doesn't seem right. Maybe a problem in the way "additional-bindings" work?

[hexfusion]

Yeah something is amiss, it works but not consistently. Possible upstream bug, but not sure.

[gyuho]

@hexfusion Can we also update changelogs?

[hexfusion]

@gyuho sure would you mind reviewing #9460 we need to address semaphore timeouts. Also I have experienced this same code above, passing and then failing Lease tests so more review is required before merge. #9460 will help this effort, thanks!

[hexfusion]

@akashbaid, I find the below to be stable and produce proper Swagger. WIll update PR and tests once #9460 gets merged.

  // LeaseLeases lists all existing leases.                                              
  rpc LeaseLeases(LeaseLeasesRequest) returns (LeaseLeasesResponse) {                    
      option (google.api.http) = {                                                       
        post: "/v3/lease/leases"                                                         
        body: "*"                                                                        
        additional_bindings {                                                            
            post: "/v3/kv/lease/leases"                                                  
            body: "*"                                                                    
        }                                                                                
    };                                                                                   
  }

[skydoctor]

That's great! Thanks for looking into it.

[hexfusion]

@gyuho updated release notes and tweaked lease tests for coverage. Should be good on greens.

[gyuho]

lgtm. thanks @hexfusion !