What metrics would you focus on as the PM for YouTube?

Metrics to focus on as a PM of Youtube -

Product Description - Videos - created by people for the people - on all kind of things, activities etc.
Business Model - Advertisement based Revenue

High Level Goals - a) To minimize churn and b) To increase profitability

If we go one step further, since the product is in mature stage, Youtube would need to consider keeping up-to-date with market trends also as primary goal to ensure it can act on time to maintain its userbase and thus profitability.
Market Trends can be in form of
a) Change in user behaviour - Session length, Trending Content, Video Duration, Tolerance to Advertisement
b) Change in user stickiness - Launch of competitive offerings, Profitability
c) Change in technical landscape - Opportunity for better solutions, channels through which content is consumed - Web/Mobile/TV?

With clarity on high level goals, we can create user groups and then define the metrics based on their role and activities that would be important in reaching the primary goals mentioned above.

User groups:
a) Content Creators
b) Content Consumers
c) Advertisers

Let's consider Content Creators only for sake of simplicity -
Metrics to be monitored inline with primary goals:

1) Change in User behavior -
    a) Change in # of content created per week/month by the user/overall
    b) Change in # of content created based on popularity (follows, likes, comments, shares)
    c) Cross collaboration between multiple content creators - # of content created with collaboration/ # of total content created
    d) Trending/Viral Topics - # of new content creators starting with trending/viral topics, # of existing content creators incorporating trending content

2) Change in User stickiness -
    a) Change in # of content creators per week/month - Deactivation of content creator accounts, Active/Inactive
    b) Change in # of valuable content creators per week/month - The ones whose views, likes, shares, comments, followers exceed a threshold
    c) Advertisement Revenue - % of creators who enable their channel for monetization within first week/month/threshold value
    d) Youtube Premium Revenue - # of creators whose content is being watched in youtube premium

3) Change in technical landscape/functionalities -
    a) % of content creators adopting to new advancements/tools within first week/month/At all

All of these are quite relevant for the current stage of Youtube's product lifecycle as they are aligned with primary goals. Examples for some other  metrics with slightly lesser priorities can be - # of content creators based on topics/categories/geographies/Age group etc. considering product has matured enough in terms of category/geography/demographic expansion.

Thanks. Please feel free to share feedback.