Router v2: Gauge are not exporter to prometheus

[yanns]

Describe the bug

I'm migrating some rust plugins from router v1 to router v2.
For metrics, I'm using the new approach with:

let meter = meter_provider().meter("apollo/router");

I'm using counter, histograms and gauge.
Counters and histograms are exported in /metrics used by prometheus, as expected.
Gauges are not exported.

I still don't understand what is wrong only with Gauges.
By looking at the code, I've found it that Gauge is being commented out in

router/apollo-router/src/plugins/telemetry/config_new/instruments.rs

Line 1281 in 4c7114d

// Gauge,

, but I'm not sure it's related.

Desktop (please complete the following information):

• Version 2.0.0

[yanns]

Here is a test probably reproducing the issue #6860

[yanns]

In 911780c, I've tried to add support for SyncGauge, but the test is still failing.

[yanns]

I could finally fix by adding b76d71b