title: PromQL if/else like expression
body: <p>I'm trying to calculate easter sunday in PromQL using <a href="https://en.wikipedia.org/wiki/Computus#Gauss%27s_Easter_algorithm" rel="noreferrer"> Gauss's Easter algorithm</a> (I need to ignore some alert rules on public holidays).</p>
  <p>I can calculate the day, but I'm having a problem with the month as I need something like an if/else expression. My recording rule easter_sunday_in_april returns 1 if eastern is in april and 0 if it is in march.</p>
  <p>(How) can I express the following in PromQL?</p>
  <pre><code>if(easter_sunday_in_april &gt; 0)
      return 4
  else
      return 3
  </code></pre>
  <p>For the sake of completeness, I attach my recording rules here:</p>
  <pre><code>- record: a
      expr: year(europe_time) % 4
  
    - record: b
      expr: year(europe_time) % 7
  
    - record: c
      expr: year(europe_time) % 19
  
    - record: d
      expr: (19*c + 24) % 30
  
    - record: e
      expr: (2*a + 4*b + 6*d + 5) % 7
  
    - record: f
      expr: floor((c + 11*d + 22*e)/451)
  
    - record: easter_sunday_day_of_month_temp
      expr: 22 + d +e - (7*f)
  
  
    - record: easter_sunday_day_of_month_in_april
      expr: easter_sunday_day_of_month_temp &gt; bool 31
  
    - record: easter_sunday_day_of_month
      expr: easter_sunday_day_of_month_temp % 31
  </code></pre>