Providing option to disable labels in prometheus output for string fields

[shaheed121]

#3350 introduced a change where all string fields are turned to labels. However in certain cases where string value changes, it introduces a new metric due to the change in label value.

Hence this will provide an option to disable as well as enable only for certain string values. The default behavior doesn't change with this commit.

[danielnelson]

What about using the measurement filtering options to provide this using taginclude/tagexclude and fielddrop/fieldpass?

[shaheed121]

@danielnelson Yes, I could do that. But I would have to know the list or patterns of tags beforehand. Instead if I disable converting string fields to labels globally, then I don't have to fiddle with all input plugins.

So what do you think about having atleast an option to disable this feature like how we had before 1.5.0 release?

[danielnelson]

That makes sense, could you provide this in a similar way to how we we do it in the docker plugin with docker_label_include and docker_label_exclude? Maybe we call it string_label_include and string_label_exclude.

[shaheed121]

@danielnelson To keep it consistent across output plugins, I thought of doing something like string_as_state in Riemann output plugin. So have added string_as_label and set default as true to make sure that it exhibits the current behavior. The reason I didn't go ahead with string_label_include and string_label_exclude because it would be redundant since it can also be managed by using Measurement filtering like you said.

So this change should provide options to enable/disable string metrics to labels globally and then include/exclude certain labels using measurement filtering.