Use specific exception for duplicate timeseries

[kajinamit]

Use sub-class of ValueError instead of ValueError, so that we can distinguish issues caused by wrong input (like invalid name format) from duplicate metrics being registered into the same registry.

[csmarchbanks]

Just curious what you would do when you encounter a duplicate name vs a different ValueError? I am not opposed to this if it is helpful but would like to understand as well/not encourage bad practices.

[kajinamit]

Just curious what you would do when you encounter a duplicate name vs a different ValueError? I am not opposed to this if it is helpful but would like to understand as well/not encourage bad practices.

We are currently using this client to export metrics of virtual machines running on a specific hypervisor. In this use case we have to register metrics dynamically, because virtual machines might be created/deleted/moved anytime. To achieve the dynamic detection, we check all virtual machines in a host and register metrics for them periodically, but we can't safely check if the virtual machine metrics were registered in the previous cycle without this. (For now we inspect _names_to_collectors before calling register but that's requires external access to a private item, which I want to get rid of)