If I add additional metrics for Prometheus/Grafana, the default metrics are no longer populated

I added some additional metrics, e.g. related to fee rate, and found that the default metrics were no longer populated.

That is, in specifying metrics such as the following in my `network.yaml`, the default Grafana dashboard no longer had any data. I was expecting and metrics I specified to be in addition/additive to the default metrics

```
metrics: fee_estimate_1=estimatesmartfee(1)["feerate"] fee_estimate_2=estimatesmartfee(2)["feerate"] fee_estimate_6=estimatesmartfee(6)["feerate"] fee_estimate_24=estimatesmartfee(24)["feerate"]
```

So if the user wishes to add additional metrics and maintain the default metrics, they must currently add the default metrics to their metrics specification:

```
metrics: blocks=getblockcount() inbounds=getnetworkinfo()["connections_in"] outbounds=getnetworkinfo()["connections_out"] mempool_size=getmempoolinfo()["size"] fee_estimate_1=estimatesmartfee(1)["feerate"] fee_estimate_2=estimatesmartfee(2)["feerate"] fee_estimate_6=estimatesmartfee(6)["feerate"] fee_estimate_24=estimatesmartfee(24)["feerate"]
```

Should any metrics provided by the user be additive to the default metrics? Or is the current behaviour ok - user has full control over what metrics are collected. The main downside of the latter is that the user will have empty Grafana Dashboards:

Start by tracing how the metrics specification in network.yaml is parsed and how default metrics are passed to Prometheus and the Grafana dashboard. Compare behavior with and without user-defined metrics; done means custom metrics are retained alongside the defaults and the default dashboard remains populated.

Written by the indexing model from the issue text.