Flexible metrics label customization without building a custom extproc

[mathetake]

Description:

Currently, the only way to add additional metrics label is to use the custom metrics interface and rebuild your own external processor binary. That works for advanced users who are in any ways willing to rebuild the ext proc container and deploy by themselves. However, there's clearly a huge hurdle for a lot people, especially people who start looking at this project or maybe are not familiar with Go, or want to use the vanilla upstream artifacts.

100% of the use case i have heard about this metrics customization so far is about "adding additional labels from headers". Hence, headers is the only context that can be used with the custom metrics interface right now.

So I propose that we add a configuration to allow users to specify header names to be added to the metrics. More precisely, I am proposing to add --metrics-header-names to the extproc command line flags where users specify header names to be added to the metrics.

For example, with --metrics-header-names=foo,bar, the extproc will add the labels corresponding to the headers whose names are either foo or bar.