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The host label comes from the request Host header, so a server reachable without a filtering proxy gets a new set of series for every Host value a client sends. It is a default label and cannot be removed from the label set, but a LabelFuncs entry returning an empty value drops it in Prometheus. Document that next to LabelFuncs. Co-Authored-By: Claude Opus 5.5 <[email protected]>
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Summary
Documentation only. Extends the
LabelFuncsdescription with how to drop the defaulthostlabel, and why one mightwant to.
Why
The
hostlabel is taken from the requestHostheader, which the client sets freely. Behind a proxy that onlyforwards known hosts this is harmless. When the server is reachable directly, every distinct
Hostvalue a clientsends creates a new set of series (41 per value with the default metrics), the same unbounded growth as raw paths in
the
urllabel.hostis one of the four default labels and cannot be removed from the label set. It can, however, be blanked throughLabelFuncs, and Prometheus treats a label with an empty value as absent(data model):
This works today, but nothing points to it. Checked against the current code: the exported line becomes
echo_requests_total{code="200",host="",method="GET",url="/ok"} 1.Not changed
The default stays:
hostis useful for servers that serve several hosts. If you'd prefer an explicit option to leavethe label out (for example
DisableHostLabel), I can change this PR to add one instead.Co-Authored-By: Claude Opus 5.5 [email protected]