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`

```jsonc
"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