fix #111 (#114)

Author: findmyway

src/sdk/src/metric/metric_reader.jl

@@ -59,13 +59,17 @@ MetricReader() = MetricReader(global_meter_provider(), ConsoleExporter())
 
 For async instruments in `r`, their callbacks will be executed first before reading all the metrics.
 """
-function (r::MetricReader)()
+function (r::MetricReader{<:MeterProvider})()
     for ins in r.provider.async_instruments
         ins()
     end
     export!(r.exporter, metrics(r.provider))
 end
 
+function (r::MetricReader)()
+    export!(r.exporter, metrics(r.provider))
+end
+
 # ??? close provider?
 Base.close(r::MetricReader) = close(r.exporter)
 

src/sdk/test/metric.jl

@@ -247,3 +247,10 @@ end
     @test length(e) == 1
     @test e.pool[1].name == "Y"
 end
+
+@testset "#111" begin
+    p = OpenTelemetryAPI.DummyMeterProvider()
+    e = InMemoryExporter()
+    r = MetricReader(p, e)
+    r()
+end