prometheus codec time param parsing docstrings and emulating upstream prometheus behavior

[francoposa]

re-reviewed these utils against the upstream prometheus implementation.

No behavior changes, just docstrings referencing the canonical implementation, and updating the signature of the instant query decoder to take a time.Now func like upstream does.

What this PR does

Which issue(s) this PR fixes or relates to

Fixes #

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.

[leizor]

I don't feel particularly strongly about this, but what do you think about a direct link?

Suggested change

	// emulating the logic in prometheus/prometheus/web/api/v1#API.query_range.
	// emulating the logic in https://github.com/prometheus/prometheus/blob/0763ec841bdc75d163a123dbffd05b12ff0500f1/web/api/v1/api.go#L505-L582.

[leizor]

(same for the other ones)

[dimitarvdimitrov]

neat, we can probably remove this piece of code after merging your change. It's simpler to think about this as part of the decoding logic more than as part of the generic request handling logic

mimir/pkg/frontend/querymiddleware/roundtrip.go

Lines 422 to 439 in e14de95

	func defaultInstantQueryParamsRoundTripper(next http.RoundTripper) http.RoundTripper {
	return RoundTripFunc(func(r *http.Request) (*http.Response, error) {
	if IsInstantQuery(r.URL.Path) && !r.Form.Has("time") && !r.URL.Query().Has("time") {
	nowUnixStr := strconv.FormatInt(time.Now().Unix(), 10)
	q := r.URL.Query()
	q.Add("time", nowUnixStr)
	r.URL.RawQuery = q.Encode()
	// If form was already parsed, add this param to the form too.
	// (The form doesn't have "time", otherwise we'd not be here)
	if r.Form != nil {
	r.Form.Set("time", nowUnixStr)
	}
	}
	return next.RoundTrip(r)
	})
	}

[dimitarvdimitrov]

remove this piece of code after merging your change

I opened #7026 to do the clean up