avalanche overrides remote url path

[watson28]

Since avalanche migrated to prometheus/client_golang's exp/api/remote package, the path component of --remote-url is no longer respected. The library appends the default path api/v1/write to whatever path the user provides, making it impossible to target endpoints that use a different path — most notably Thanos Receiver, which expects /api/v1/receive.

Reproduction

services:
avalanche:
image: quay.io/prometheuscommunity/avalanche:main
command:
- --remote-url=https://user:pass@thanos-receiver.example.com/api/v1/receive
- --gauge-metric-count=100
- --series-count=10

Expected: POST requests to https://thanos-receiver.example.com/api/v1/receive.
Actual: POST requests to https://thanos-receiver.example.com/api/v1/receive/api/v1/write → 404 Not Found from Thanos Receiver.

Root cause

In metricsgen/write.go (lines 115-119), remote.NewAPI is called without WithAPIPath:

remoteAPI, err := remote.NewAPI(
cfg.URL.String(),
remote.WithAPIHTTPClient(httpClient),
remote.WithAPILogger(logger.With("component", "remote_write_api")),
)

prometheus/client_golang/exp/api/remote/remote_api.go then defaults o.path to "api/v1/write" (line 81) and joins it onto the parsed URL (line
151):

parsedURL.Path = path.Join(parsedURL.Path, o.path)

So any path supplied in --remote-url is treated as a prefix, not the final path.

The help claims "By default, path is set to api/v1/write," which now reads as misleading — the path provided in the URL is silently overridden, not used as a default.

Workaround

Pin to quay.io/prometheuscommunity/avalanche:v0.6.0, which uses http.NewRequest("POST", c.config.URL.String(), ...) directly
(metrics/write.go:309) and respects the URL path.