bug: promslog panics if a user sets a log attribute with a key of source

[tjhop]

This issue is for tracking purposes, @SuperQ and I are discussing things further in slack.

---

In our slog implementation, we always instruct slog to emit source code info of the calling log statement:
https://github.com/prometheus/common/blob/main/promslog/slog.go#L183

We also do some default formatting of the resulting source code position info, both in the default implementation and the legacy go-kit styled implementation:
https://github.com/prometheus/common/blob/main/promslog/slog.go#L77-L79
https://github.com/prometheus/common/blob/main/promslog/slog.go#L54-L63

As mentioned in slog's docs, the AddSource field adds a key-value pair onto the logger keyed by SourceKey, which is a constant value of source:
https://pkg.go.dev/log/slog#pkg-constants

We key on that SourceKey in our ReplaceAttr funcs that we use for formatting log lines, and when an attribute is found with a key equal to SourceKey, it extracts the any from the value and casts it to an *slog.Source. We do not check the error from the type cast, as slog always adds source info as an attribute with a key of SourceKey and value of slog.Source.

This implementation is naive, as users may add their own attributes with a key of source, causing promslog to panic:
SuperQ/chrony_exporter#107

Putting aside the usefulness/validity of duplicate keys in a structured log message, this is quite the landmine for users to run into, and promslog should handle it more gracefully.

[SuperQ]

We could do something similar to what we do for labels in Prometheus. We can have a set of reserved keys and rename duplicate keys to logged_KEY. So if someone logs source, it would become logged_source.