What this PR does

Adds use_fips_sts_endpoint to the sigv4 block in docs/configuration/configuration.md, including a short description, and minimum version note, The field enables use of the FIPS-compliant AWS STS endpoint when assuming roles.

• Field added in both occurrences of the shared <http_config> → sigv4 section in the page.
• Notes that Prometheus v2.54.0+ is required (first release with prometheus/common v0.55.0).

Rationale / background

Environment variable–based selection (AWS_USE_FIPS_ENDPOINT) no longer drives STS FIPS usage for this path; the documented way is the config knob use_fips_sts_endpoint. This closes the docs gap so users can discover the option directly from the Prometheus configuration page.

References:

• Feature introduced in prometheus/common and consumed by Prometheus in v2.54.0. link
• Related discussion: AWS_USE_FIPS_ENDPOINT no longer works for this case.
• Operator surfaced the option in prometheus-operator and adjusted wording shortly after; this change aligns the server docs.

Example

remote_write:
  - url: https://aps-workspaces.us-east-1.amazonaws.com/workspaces/XXXX/api/v1/remote_write
    http_config:
      sigv4:
        region: us-east-1
        role_arn: arn:aws:iam::111122223333:role/prom-remote-write
        use_fips_sts_endpoint: true

Which issue(s) does the PR fix:

• Fixes Add use_fips_sts_endpoint to sigv4 documentation. #17301

Related

• AWS_USE_FIPS_ENDPOINT environment variable not being supported by prometheus 3.4.1 though it was supported by 2.55.1 #16752
• Add SigV4 FIPS STS endpoint config common#649
• feat: add support for configuring use_fips_sts_endpoint in sigv4 config prometheus-operator/prometheus-operator#7987, Allow remote read to not add external labels #7992

Does this PR introduce a user-facing change?

NONE

Checklist

• Changes limited to documentation (docs/configuration/configuration.md).
• DCO sign-off on commits (git commit -s).
• Comments follow style (capitalized, end with a period).
• Kept wording consistent with nearby sigv4 entries.

Reviewers to ping (suggested): @roidelapluie @metalmatze