support multiple instrument callback

src/api/src/metric/instruments.jl

@@ -4,7 +4,8 @@ export Measurement,
     Histogram,
     ObservableGauge,
     UpDownCounter,
-    ObservableUpDownCounter
+    ObservableUpDownCounter,
+    ObservableMultiInstrument
 
 """
     Measurement(value, [attributes=BoundedAttributes()])
@@ -225,3 +226,23 @@ struct ObservableUpDownCounter{T,F} <: AbstractAsyncInstrument{T}
         c
     end
 end
+
+
+"""
+    ObservableMultiInstrument(callback, instruments)
+
+The number of measurements returned by the `callback` should be the same as the number of `instruments`.
+See also [#112](https://github.com/oolong-dev/OpenTelemetry.jl/issues/112)
+"""
+struct ObservableMultiInstrument <: AbstractAsyncInstrument{Any}
+    callback::Any
+    instruments::AbstractVector{<:AbstractSyncInstrument}
+end
+
+function (ins::ObservableMultiInstrument)()
+    measurements = ins.callback()
+    @assert length(measurements) == length(ins.instruments) "length mismatch"
+    for (x, m) in zip(ins.instruments, measurements)
+        x(m)
+    end
+end

src/api/src/trace/tracer_basic.jl

@@ -94,6 +94,7 @@ Base.getindex(s::TraceState, key) = s.kv[key]
 Base.haskey(s::TraceState, key) = haskey(s.kv, key)
 Base.haskey(s::TraceState, key::String) = haskey(s, Symbol(key))
 Base.length(s::TraceState) = length(s.kv)
+Base.isempty(s::TraceState) = isempty(s.kv)
 
 function Base.show(io::IO, ts::TraceState)
     for (i, (k, v)) in enumerate(pairs(ts.kv))

src/api/test/metric.jl

@@ -45,5 +45,10 @@
             rand(1:10)
         end
         obs_up_down_counter()
+
+        obs_multi_ins = ObservableMultiInstrument([counter, up_down_counter]) do
+            (rand(1:10), rand(-10:10))
+        end
+        obs_multi_ins()
     end
 end

src/api/test/trace.jl

@@ -21,7 +21,7 @@
 
             @test isempty(parse(TraceState, ""))
             @test !isempty(parse(TraceState, "a=b,c=d"))
-            @test !isempty(parse(TraceState, "a=b,ss=dwdw=dwdwd,c=d"))
+            @test length(parse(TraceState, "a=b,ss=dwdw=dwdwd,c=d")) == 2
         end
     end