[stats] lazyinit ClusterInfo::trafficStats() (#23921)

Commit Message: LazyInit ClusterInfo::trafficStats().

Additional Description:
3rd PR for https://github.com/envoyproxy/envoy/issues/23575, this is stack upon 2nd PR https://github.com/envoyproxy/envoy/pull/23907

With 100K clusters, we are seeing ~1.5 GBs less RAM usage with this PR.
Kudos to @jmarantz for the data:

|                      | Clean client     | in-MB       | Defferred | Diff       | Diff %           | Defferred-Fulfilled | Diff-in-MB (fulfilled - Clean) |
|----------------------|------------|-------------|-------------------|------------|-----------------|-----------------------------|-----------------------------|
| allocated            | 4561550208 | 4350.233276 | 2886860656        | 1674689552 | 36.71316714    | 4565167632                  | 3.44984436                  |
| heap_size            | 5303697408 | 656         | 3443523584        | 1860173824 | 35.07315144    | 5146411008                  | -150                        |
| pageheap_unmapped    | 687865856  |             | 501219328         | 186646528  | 27.13414634    | 524288000                   | -156                        |
| pageheap_free        | 22921216   | 21.859375   | 23109632          | -188416    | -00.8220157255 | 22257664                    | -0.6328125                  |
| total_thread_cache   | 1718288    | 1.638687134 | 4197032           | -2478744   | -144.2566089    | 4833576                     | 2.970970154                 |
| total_physical_bytes | 4647192158 | 4431.907804 | 2965242430        | 1681949728 | 36.19281645    | 4653479518                  | 5.99609375                  |

To reproduce, use this script to add 100K clusters to the bootstrap tmpl:
```
$ cat large_bootstrap_maker.sh
#!/bin/bash

set -u
set -e
limit="$1"

for i in $(seq 1 $limit); do
  service="zzzzzzz-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx_$i"
  echo "  - name: $service"
  echo "    connect_timeout: 0.25s"
  echo "    type: LOGICAL_DNS"
  echo "    dns_lookup_family: \"v6_only\""
  echo "    lb_policy: ROUND_ROBIN"
  echo "    load_assignment:"
  echo "      cluster_name: $service"
  echo "      endpoints:"
  echo "      - lb_endpoints:"
  echo "        - endpoint:"
  echo "            address: {socket_address: {address: google.com, port_value: 443}}"
done

```
base.tmpl:

```$ cat base.yaml
admin:
  access_log:
  - name: envoy.access_loggers.file
    typed_config:
      "@type": type.googleapis.com/envoy.extensions.access_loggers.file.v3.FileAccessLog
      path: "/dev/null"
  address:
    socket_address:
      address: "::"
      port_value: 8080
layered_runtime:
  layers:
  - name: admin
    admin_layer: {}

static_resources:
  listeners:
  - name: listener_0
    address:
      socket_address:
        address: "::"
        port_value: 0
    filter_chains:
    - filters:
      - name: http
        typed_config:
          "@type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
          stat_prefix: ingress_http
          codec_type: AUTO
          route_config:
            name: local_route
            virtual_hosts:
            - name: local_service
              domains: ["*"]
              routes:
              - match: {prefix: "/"}
                route: {host_rewrite_literal: 127.0.0.1, cluster: yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy-wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww}
  clusters:
```

Run the following commands to generate the bootstrap.
```
bash large_bootstrap_maker.sh 100000 >> base.yaml
mv base.yaml test/config/integration/100k_clusters.yaml
```
and modify the test/config/main_common_test.cc to load from test/config/integration/100k_clusters.yaml instead. Then you could run the test and observe the RAM uasage.

Risk Level: Medium (changes how cluster_info::trafficStats() are created).

Testing: existing unit tests.
Docs Changes: done
Release Notes: incluided
Platform Specific Features: n/a

Signed-off-by: Xin Zhuang <stevenzzz@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 23d6164e4405c9389fb1f6403f274942e9dd2d9e

envoy/config/bootstrap/v3/bootstrap.proto

@@ -199,7 +199,6 @@ message Bootstrap {
   repeated metrics.v3.StatsSink stats_sinks = 6;
 
   // Options to control behaviors of deferred creation compatible stats.
-  // [#not-implemented-hide:]
   DeferredStatOptions deferred_stat_options = 39;
 
   // Configuration for internal processing of stats.