Allow inc with negative, and dec with positive, values for Gauge.

src/Prometheus.jl

@@ -236,9 +236,6 @@ Increment the value of the gauge `g` with `v`.
 `v` defaults to `v = 1`.
 """
 function inc(m::Gauge, v = 1.0)
-    if v < 0
-        error("incrementing with negative value, use dec(...)?")
-    end
     @atomic m.value += v
     return nothing
 end
@@ -250,9 +247,6 @@ Decrement the value of the gauge `g` with `v`.
 `v` defaults to `v = 1`.
 """
 function dec(m::Gauge, v = 1.0)
-    if v < 0
-        error("decrementing with negative value, use inc(...)?")
-    end
     @atomic m.value -= v
     return nothing
 end

test/runtests.jl

@@ -76,13 +76,11 @@ end
     @test c.value == 1
     Prometheus.inc(c, 2)
     @test c.value == 3
-    @test_throws ErrorException Prometheus.inc(c, -1)
     # Prometheus.dec(...)
     Prometheus.dec(c)
     @test c.value == 2
     Prometheus.dec(c, 1)
     @test c.value == 1
-    @test_throws ErrorException Prometheus.dec(c, -1)
     # Prometheus.set_to_current_time(...)
     t0 = time()
     sleep(0.1)