GitHub Enterprise Server インスタンスの collectd -> collectd サーバー -> prometheus というデータフローでメトリクスを収集すると、GHES インスタンスから来たメトリクスには instance とは別に exported_instance というラベルがついていて、ここに GHES インスタンスのホスト名が入っている。
exported_instance はどのようにして付与されるのか？scrape_config の honor_labels オプションで説明されていた。
# 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 ]
ここがキモ:
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.
要するに、collectd サーバーでデータを集約してるような場合は、honor_labels: true にすべきっぽい。すると、ラベルの付け替えなどをしなくても、instance などに普通に望ましい値が入ってくれると。
