clients/pkg/promtail/client: add documentation about default values

[kevinburkesegment]

I'm trying to modify an existing binary to send logs to Loki. client.New takes a number of arguments (max line size, etc) but does not indicate what the defaults are. Trying to read through the code to find them is a bit tough. promtail --help does not report the default values or provide any guidance for line size, or streams.

  -max-line-size value
    	Maximum log line byte size allowed without dropping. Example: 256kb, 2M. 0 to disable.
  -max-line-size-truncate
    	Whether to truncate lines that exceed max_line_size. No effect if max_line_size is disabled
  -max-streams int
    	Maximum number of active streams. 0 to disable.

I can always think harder about this later, but for now I'm just trying to pick a reasonable set of defaults and would not like to think too hard about this.

One other thing, when you run promtail --help it says you are running some binary named "config-file-loader":

$ promtail --help
Usage of config-file-loader:
  -check-syntax
    	Validate the config file of its syntax
  -client.batch-size-bytes int
    	Maximum batch size to accrue before sending (deprecated). (default 1048576)
    	
   ... 	

[vash-knives]

@JStickler , can probably add the default on the --help output? (unless we can't do that since this is a loki repo) I see the deafult values on here docs directory. I can work on this if nobody took it yet.

[JStickler]

@vash-knives, the Promtail code is in the Loki repo -> https://github.com/grafana/loki/tree/main/clients/pkg/promtail

[vash-knives]

@JStickler created a PR for it. Let me know if there are any issues / comments / concerns.

This change does not address the "config-file-header" issue the OP mentioned. Will probably need a separate ticket for that as it's being pulled from a different directory (pkg\util\cfg). I'm not even sure if it's something we needed to change and if so, to what specific value.