Cos proxy is sending to prometheus targets, from only one prometheus-target relation

### Bug Description

We have several telegraf apps related to prometheus-target interface of cos-proxy.
On prometheus we could see data only from first telegraf unit. As well as within relation data
On cos proxy unit I could see data from relations to all telegraf units

### To Reproduce

Relate several telegraf units to one cos-proxy

### Environment

Charm revision 74

### Relevant log output

```shell
Unfortunately no logs to add
```

### Additional context

_No response_

No file, test, or code entry point is identified. Start by reproducing the issue with several telegraf units related through prometheus-target, then inspect how cos-proxy gathers relation data and publishes Prometheus targets. Done means every related telegraf unit is represented in the Prometheus data and relation output.

Written by the indexing model from the issue text.