Add a new flag to pings: send_if_empty

[badboy]

No description provided.

[codecov-io]

Codecov Report

Merging #528 into master will decrease coverage by <.01%.
The diff coverage is 63.63%.

@@             Coverage Diff              @@
##             master     #528      +/-   ##
============================================
- Coverage     75.93%   75.92%   -0.01%     
  Complexity      333      333              
============================================
  Files            96       96              
  Lines          5643     5649       +6     
  Branches        700      703       +3     
============================================
+ Hits           4285     4289       +4     
  Misses          842      842              
- Partials        516      518       +2

Impacted Files	Coverage Δ	Complexity Δ
...n/java/mozilla/telemetry/glean/rust/LibGleanFFI.kt	66.66% <ø> (ø)	0 <0> (ø)	⬇️
glean-core/src/lib.rs	63.1% <ø> (+0.53%)	0 <0> (ø)	⬇️
glean-core/src/metrics/ping.rs	23.07% <0%> (-4.2%)	0 <0> (ø)
glean-core/ios/Glean/Metrics/Ping.swift	90.9% <100%> (ø)	0 <0> (ø)	⬇️
...n/java/mozilla/telemetry/glean/private/PingType.kt	81.81% <100%> (+1.81%)	3 <0> (ø)	⬇️
glean-core/src/ping/mod.rs	67.66% <75%> (-0.02%)	0 <0> (ø)

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[Dexterp37]

Looks good overall, just some minor discussion points below.

[mdboom]

r+ wc