Add JSON request logging for gitaly-ruby

Part of gitlab-com/gl-infra/scalability#519 (closed)

This MR upgrade labkit-ruby to 0.13, which includes a new gRPC server middleware that writes an access log. The access log goes into a separate file gitaly_ruby_json.log in the Gitaly logging dir.

Example log line:

{"type":"gitaly-ruby","grpc.time_ms":0.221,"grpc.code":"OK","grpc.method":"Check","grpc.service":"grpc.health.v1.Health","pid":34993,"correlation_id":"f08f390f35a7a7e6c2dffa59c27b0878","time":"2020-10-20T13:42:11.385Z"}