prometheus 中 honor_labels 配置项的源码分析

# honor_labels controls how Prometheus handles conflicts between labels that are
# already present in scraped data and labels that Prometheus would attach
# server-side ("job" and "instance" labels, manually configured target
# labels, and labels generated by service discovery implementations).
#
# If honor_labels is set to "true", label conflicts are resolved by keeping label
# values from the scraped data and ignoring the conflicting server-side labels.
#
# If honor_labels is set to "false", label conflicts are resolved by renaming
# conflicting labels in the scraped data to "exported_<original-label>" (for
# example "exported_instance", "exported_job") and then attaching server-side
# labels.
#
# Setting honor_labels to "true" is useful for use cases such as federation and
# scraping the Pushgateway, where all labels specified in the target should be
# preserved.
#
# Note that any globally configured "external_labels" are unaffected by this
# setting. In communication with external systems, they are always applied only
# when a time series does not have a given label yet and are ignored otherwise.
[ honor_labels: boolean | default = false ]

func mutateSampleLabels(lset labels.Labels, target *Target, honor bool, rc []*relabel.Config) labels.Labels {
	lb := labels.NewBuilder(lset)
	if honor {
		for _, l := range target.Labels() {
			if !lset.Has(l.Name) {
				lb.Set(l.Name, l.Value)
			}
		}
	} 
       ...忽略
}

// Set the name/value pair as a label.
func (b *Builder) Set(n, v string) *Builder {
	if v == "" {
		// Empty labels are the same as missing labels.
		return b.Del(n)
	}
	for i, a := range b.add {
		if a.Name == n {
			b.add[i].Value = v
			return b
		}
	}
	b.add = append(b.add, Label{Name: n, Value: v})
 
	return b
}