fix(router, only 1.x): fix the apollo_router_session_count_total metric

[goto-bus-stop]

In #6495 I migrated this metric to an otel ObservableGauge. However the lifetime of the count guard was too short, so the value of the metric would almost always just be 0; and the lifetime of the instrument was too short, so even the wrong value of 0 would not actually be reported. This change was not part of a release so there is no user impact as long as we land this :)

Now the lifetime of the instrument is captured by all requests, so the instrument is kept even post-reload if a request is still being completed from the old schema.

Also, a test verifies that the count increments and decrements as expected.

This does not need to be ported to Router 2.0.

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

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Build ID: 9e0cc6433f744dc0a5139477

[tninesling]

LGTM! Does this need a changeset, or are you reusing one that's already on dev?

[goto-bus-stop]

Since it's just a fix to a bug that has never reached a release, I think it doesn't justify a changeset!

I might add the PR number to an existing changeset.

[router-perf]

CI performance tests

• connectors-const - Connectors stress test that runs with a constant number of users
• const - Basic stress test that runs with a constant number of users
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• enhanced-signature - Enhanced signature enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• extended-reference-mode - Extended reference mode enabled
• large-request - Stress test with a 1 MB request payload
• no-tracing - Basic stress test, no tracing
• reload - Reload test over a long period of time at a constant rate of users
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-local-metrics - Field stats that are generated from the router rather than FTV1
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• step - Basic stress test that steps up the number of users over time
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload