Record message size transmitted over action cable

What does this MR do?

Record message size transmitted with action cable.

related to #326364 (closed)

Sample payload transmitted on action cable

The data processed on ruby app is a Hash object, however, it is encoded as JSON before sent over the websocket. We have to serialize the data to measure its size precisely. For more information check #326364 (comment 543538666).

 {
     "result" => {
         "data" => {
             "issuableAssigneesUpdated" => {
                  "assignees" => {
                          "nodes" => [
                         [0] {
                                 "status" => nil,
                             "__typename" => "UserCore",
                                     "id" => "gid://gitlab/User/25",
                              "avatarUrl" => "https://www.gravatar.com/,
                                   "name" => "jaisna jaisna",
                               "username" => "jaisna",
                                 "webUrl" => "http://127.0.0.1:3000/jaisna"
                         }
                     ],
                     "__typename" => "UserCoreConnection"
                 },
                 "__typename" => "Issue"
             }
         }
     },
  "more" => true
}

Sample of metrics generated on localhost

# HELP action_cable_transmitted_bytes Multiprocess metric
# TYPE action_cable_transmitted_bytes histogram
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="+Inf"} 1
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="0.005"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="0.01"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="0.025"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="0.05"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="0.1"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="0.25"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="0.5"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="1"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="10"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="2.5"} 0
action_cable_transmitted_bytes_bucket{channel="GraphqlChannel",operation="issuableAssigneesUpdated",le="5"} 0
action_cable_transmitted_bytes_count{channel="GraphqlChannel",operation="issuableAssigneesUpdated"} 1
action_cable_transmitted_bytes_sum{channel="GraphqlChannel",operation="issuableAssigneesUpdated"} 388

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