Add an endpoint to signal_dashboard that returns report freshness

[krivard]

From RoniRos in a thread from the indicator status/backfill naming doc:

What I have in mind for 'historical availability' is something much simpler than the (admittedly very complex) notion of "what was known when". I just want to know, for each signal, how "fresh" it has been, for some simplistic definition of "fresh". Here is one definition: for a given signal and a given reporting date, report the smallest lag available for that date. ~That's 40 integers per day. Later on, if we want to get fancy, we can make that specific to each location.

(crossouts are mine; we actually have ~150 unique source+signal pairs)

For v1, rather than caching this information we should just compute it at query time.

Easiest & quickest implementation I can think of is the following:

Given: reporting_date, source, signal
Compute:

min(lag) 
WHERE issue=reporting_date 
AND source=source 
AND signal=signal

A slightly more robust (in case of reporting gaps), but slower-to-compute implementation is:

Given: reporting_date, source, signal
Compute:

min(lag) 
WHERE issue<=reporting_date 
AND source=source 
AND signal=signal 
AND time_value=(
    max(time_value) where issue<=reporting_date` AND source=source AND signal=signal
)

┆Issue is synchronized with this Asana task by Unito

[RoniRos]

Thanks.
150 signals. Oh Mine. I presume the majority are survey marginals?

[RoniRos]

Just to clarify: once this endpoint is added, we will add a way to visualize this summary statistics for all (desired) signals in the dashboard - correct?

[krivard]

once this is added, we'll track visualization in a separate task