Feature request: I'd like better access to the Ruby Spanner RPC list_sessions method to support Emulator session transaction resetting.

Background

Facts:

1. The Spanner local emulator only permits one transaction to be open at a time.
2. Opening a transaction and then crashing or killing a program while a transaction is open can result in the client going away and the stale transaction left open until the emulator is restarted.

Annoying, because rebuilding large (100+ table, 300+ index, 1000s of columns) database from scratch can take up to 30 seconds. It'd be much faster to just clear stale open transactions for detached sessions.

Fast forward, I figured out how to do this with the existing library, but it required this monkey patch:

module Google
  module Cloud
    module Spanner
      class Service
        # add a missing list_sessions method
        # @param database [String] in the form of a full Spanner identifier like
        #                          "project/.../instance/.../database/..."
        def list_sessions(database:, call_options: nil, token: nil, max: nil)
          opts = default_options call_options: call_options
          request = {
            database: database,
            page_size: max,
            page_token: token
          }
          paged_enum = service.list_sessions request, opts
          paged_enum.response
        end
      end
    end
  end
end

And then some egregious private API calling to:

• list instances on a project
• list databases on an instance
• list sessions on a database
• open and immediately roll back a transaction on a specific project/instance/database/session

So, it'd be cool to have list_sessions as a first class method on whatever class or classes where it makes sense.