Add a WithKeyFormatter option

[MarkusFreitag]

Hi @rodaine ,
today I stumbled across this library and would like to propose another protoslog.Option.
I have the need to generate the key for a field based on its type. Please give me some feedback whether you would accept this addition to your library.

Lets assume there are the following proto messages.

message Event {
  google.protobuf.Any payload = 1;
}
message PayloadA {
  string id = 1;
}
message PayloadB {
  uint64 id = 1;
}

Logging an incoming event using your library will produces the following log entries. For simplicity log attributes like time and level are removed.

{"msg":"incoming event","event":{"payload":{"@type":"type.googleapis.com/protobuf.PayloadA","id":"fc2ea33d-59bf-4902-8b55-2ac334f61704"}}}
{"msg":"incoming event","event":{"payload":{"@type":"type.googleapis.com/protobuf.PayloadB","id":123}}}

These logs are collected and pushed into a logstorage. When it parses the log entries, having two equal keys (event.id from both payloads) with different types will result in an error. Using the added option, I can provide a function that will extend the key with a suffix based on the actual payloads type. Afterwards the log entries would look like this:

{"msg":"incoming event","event":{"payload[protobuf/PayloadA]":{"@type":"type.googleapis.com/protobuf.PayloadA","id":"fc2ea33d-59bf-4902-8b55-2ac334f61704"}}}
{"msg":"incoming event","event":{"payload[protobuf/PayloadB]":{"@type":"type.googleapis.com/protobuf.PayloadB","id":123}}}

If you consider to accept this PR, I will add some tests using the above mentioned example.

[rodaine]

Hi, @MarkusFreitag! Out of curiosity, which log storage utility do you use?

[rodaine]

Also, this PR is great, just missing tests!

[MarkusFreitag]

Hi, @MarkusFreitag! Out of curiosity, which log storage utility do you use?

The logs are pushed into OpenSearch.

Also, this PR is great, just missing tests!

I've added some unit-tests, please led me know whether anything else is needed.

[rodaine]

Thanks, @MarkusFreitag!