MoQ stats gauge correction

[akash-a-n]

removed unnecessary stats
correctly tracks the number of active publishers

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This change is 

[akash-a-n]

I have removed a bunch of namespace gauges that were inaccurately tracking the number of active publishNamespace and subscribeNamespace messages.
I have fixed a double decrement bug for pubActiveSubscribers -- (number of subscribers when session is a publisher)
I have fixed the non-decrementing issue with subActivePublishers -- (number of publishers when session is a subscriber)

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[gmarzot]

how does this PR relate to: openmoq/moxygen#148

independent?

[akash-a-n]

The third fix - for subActivePublishers needed changes in in both the layers moqx relay and in the underlying moxygen. #148 corresponds to the underlying change.

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[peterchave]

Has a fix for moqx_quicActiveStreams been looked at?
That guage appears to either not be incrementing or double decrementing?

[peterchave]

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[akash-a-n]

I'll take a look at moqx_quicActiveStreams

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[akash-a-n]

Fixed it in #256

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[afrind]

@akash-a-n I rebased/force-pushed this for you.

Claude noticed:

Potential bug — onSubscribeError decrements without a prior increment:

  void MoQStatsCollector::SubscriberCallback::onSubscribeSuccess() {
    ++parent_.subActivePublishers_;   // incremented only on success
  }

  void MoQStatsCollector::SubscriberCallback::onSubscribeError(...) {
    --parent_.subActivePublishers_;   // decremented on error — but was never incremented!
  }

onSubscribeSuccess and onSubscribeError are mutually exclusive terminal outcomes of a SUBSCRIBE. If a subscribe
fails (error, not success), subActivePublishers_ will go negative. The decrement on onSubscribeError should be
removed.

--

Potential double-decrement: Both onUnsubscribe and onPublishDone in SubscriberCallback decrement
subActivePublishers_. If the upstream always sends PUBLISH_DONE after an unsubscribe (as the
PublisherCallback::onUnsubscribe comment suggests is the case on the pub side), this would double-decrement for
the same subscription end. Worth verifying which events actually fire together on the subscriber side.

This sounds strange - pretty sure moxygen doesn't fire publshDone after the app unsubscribes