Add tests covering 0 value passed to set() and observe()

Updates #172

prometheus_test.lua

@@ -328,6 +328,15 @@ function TestPrometheus:testCounters()
   luaunit.assertEquals(self.dict:get('metric2{f2="v2",f1="v1"}'), 4)
   luaunit.assertEquals(ngx.logs, nil)
 end
+function TestPrometheus:testGaugeSet()
+  self.gauge1:set(100)
+  luaunit.assertEquals(self.dict:get("gauge1"), 100)
+  self.gauge1:set(0)
+  luaunit.assertEquals(self.dict:get("gauge1"), 0)
+  self.gauge1:set(-5)
+  luaunit.assertEquals(self.dict:get("gauge1"), -5)
+  luaunit.assertEquals(self.dict:get("nginx_metric_errors_total"), 0)
+end
 function TestPrometheus:testGaugeIncDec()
   self.gauge1:inc(-1)
   luaunit.assertEquals(self.dict:get("gauge1"), -1)
@@ -598,7 +607,19 @@ function TestPrometheus:testLatencyHistogram()
   luaunit.assertEquals(self.dict:get('l2_bucket{var="ok",site="site1",le="Inf"}'), 2)
   luaunit.assertEquals(self.dict:get('l2_count{var="ok",site="site1"}'), 2)
   luaunit.assertEquals(self.dict:get('l2_sum{var="ok",site="site1"}'), 0.151)
+
+  -- test observing a zero value
+  self.hist1:observe(0)
+  self.p._counter:sync()
+  luaunit.assertEquals(self.dict:get('l1_bucket{le="00.300"}'), 1)
+  luaunit.assertEquals(self.dict:get('l1_bucket{le="00.400"}'), 3)
+  luaunit.assertEquals(self.dict:get('l1_bucket{le="00.500"}'), 3)
+  luaunit.assertEquals(self.dict:get('l1_bucket{le="Inf"}'), 3)
+  luaunit.assertEquals(self.dict:get('l1_count'), 3)
+  luaunit.assertEquals(self.dict:get('l1_sum'), 0.75)
+
   luaunit.assertEquals(ngx.logs, nil)
+  luaunit.assertEquals(self.dict:get("nginx_metric_errors_total"), 0)
 end
 function TestPrometheus:testLabelEscaping()
   self.counter2:inc(1, {"v2", "\""})