PIP-235: Add metric for subscription backlog size

### Motivation

Now we have `pulsar_storage_backlog_size` for topic backlog size, user can create an alarm rule like `pulsar_storage_backlog_size > THRESHOLD`, typically this alarm is going to notify corresponding subscription owner, but it need extra process to identify subscriptions that backlog size exceed the threshold. So we could add a new metric for subscription back size.

#9302 Added subscription backlog size info for topic stats and a configuration `exposeSubscriptionBacklogSizeInPrometheus` (default false) for topic metrics, but the latter was unimplemented.

### Goal

New metric `pulsar_subscription_back_log_size`

### API Changes

Since `0` is a valid value for subscription backlog size, `change subscriptionStats.backlogSize to -1 from 0` if request the stats of topic with `subscriptionBacklogSize=false`.

### Implementation

If and only if `exposeSubscriptionBacklogSizeInPrometheus` is enabled, expose the metric as `pulsar_subscription_back_log_size`

### Alternatives

_No response_

### Anything else?

_No response_

Start by reviewing #9302 and the topic-stats path that introduced subscription backlog size, then trace the exposeSubscriptionBacklogSizeInPrometheus configuration into Prometheus metric generation. Done means the pulsar_subscription_back_log_size metric appears only when enabled, and subscriptionStats.backlogSize is -1 rather than 0 when subscription backlog stats are not requested.

Written by the indexing model from the issue text.