feat(slack): Add metrics for slack reinstall nudge outcomes

[alexsohn1126]

we send "nudges" -- a short CTA after alert messages -- for a subset of alerts sent to Slack channels to either update their slack app installation, or to remind them they can @ mention sentry anywhere to start debugging their system.

Let's add some metrics for this nudge so we know when we send the nudge vs. when we do not.

also see:
#118288

[sentry]

Bug: The function returns early when the feature flag is disabled but fails to increment a metric, creating an observability gap in tracking nudge outcomes.
_{Severity: LOW}

Suggested Fix

Increment a new metric before the early return when the feature flag organizations:slack-reinstall-nudge-on-issue-alert is disabled. This will provide visibility into this specific outcome, aligning with the other metric tracking in the function.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/sentry/integrations/slack/utils/nudge.py#L19-L20

Potential issue: The function `should_send_nudge_for_reinstall` returns early if the
feature flag `organizations:slack-reinstall-nudge-on-issue-alert` is disabled, but this
exit path does not increment a metric. Other paths, such as skipping due to a random
check or weekly limit, do have corresponding metrics. This omission creates an
observability gap, making it difficult to distinguish why a nudge was not sent. It
prevents monitoring systems from tracking which organizations have the feature disabled,
which goes against the pull request's stated goal of adding comprehensive metrics for
nudge outcomes.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[alexsohn1126]

not caring about feature flag is okay imo