option to not include _entry in loki.pack

[yosefy]

Request

sometimes _entry value is not needed and it is too long
so may be we can add parameter to not include it in output json in stage.pack

thanks

Use case

i am parsing mysql slow query log which is pretty long and not beautiful lines
so i add the mysql query itself to query field with pack

but i really don't need it twice because it becomes almost unreadable this way

[ngc4579]

I second that. We're processing complex log lines from Java applications in JSON format; after applying a series of processing stages extracting structured metadata, I'd like to re-pack the remaining actual log content (message and potentially exceptions and call stack) into a more lightweight JSON object without repeating the already extracted structured metadata into an _entry element.

[ngc4579]

I've forked the repo and tried to prepare a PR... but actually I'm not quite sure if this is a simple thing to achieve. It all depends on this: can the logproto.Entry.Line contain null as the value of the logqlmodel.PackedEntryKey (_entry), such as shown here:

Entry: logproto.Entry{
    Timestamp: time.Unix(1, 0),
    Line: "{\"foo\":\"bar\",\"" + logqlmodel.PackedEntryKey + "\":null}",
},