Add custom stat through configuration

Title: Add a custom, static stat to Envoy’s stat output through configuration

Description: I want to be able to add a stat (metric) to envoy through configuration. For example a configuration of stat name 'foo' with fixed_value '1' would then be output a gauge like foo 1

          "stats_config": {
              "stats_tags": [
                  {
                      "tag_name": "environment",
                      "fixed_value": "prod"
                  }
              ],
# proposed new config key:
              "static_stats": [
                {
                   "stat_name": "foo",
                   "fixed_value": 1,
                }
              ]
          },

Above is an idea on where to put it, adjust the StatsConfig object https://www.envoyproxy.io/docs/envoy/v1.32.1/api-v3/config/metrics/v3/stats.proto.html#config-metrics-v3-statsconfig