helm: Log access-policy and token names in GEM query-frontend

[narqo]

What this PR does

r308 allows GEM gateway to enable passing the access-policy-name and token-name to mimir backends with the request headers. This enables GEM customers to get a better insight in who is running a query, by logging these headers in the query-frontend query stats. This PR enables updates the Helm chart, enabling this by default.

Below is an example for it looks in the query-frontend logs:

% kubectl -n test-monitoring logs deployments/mmr-mimir-query-frontend
...
ts=2024-09-20T10:17:03.035087976Z caller=handler.go:383 level=info
user=tenant-1 
msg="query stats" component=query-frontend 
method=GET path=/prometheus/api/v1/query 
route_name=prometheus_api_v1_query 
user_agent=curl/8.7.1 
status_code=200 
···
header_x_access_policy_name=test-access-policy-1 
header_x_token_name=test-access-policy-1-token-1 
status=success

That is, in addition to the user=tenant-1 that query-frontend logs already, we now log the following:

• header_x_access_policy_name=test-access-policy-1 — the access policy name
• header_x_token_name=test-access-policy-1-token-1 — the full token name

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.