mtypes: Add test to reproduce 0 divide error

This can be seen when the input doesn't have any summary metrics, as in
this minimal example:

    $ ./mtypes < /dev/null -avalanche-flags-for-adjusted-series 10

    ./mtypes < /dev/null -avalanche-flags-for-adjusted-series 10
    Metric Type    Metric Families    Series (adjusted)    Series (adjusted) %        Average Buckets/Objectives
    ---            ---                ---                  ---                        ---
    *              0                  0 (0)                100.000000 (100.000000)    -

    Avalanche flags for the similar distribution to get to the adjusted series goal of: 10
    panic: runtime error: integer divide by zero

    goroutine 1 [running]:
    main.computeAvalancheFlags(0x8a55a0?, {0x0, 0x0, 0x0, 0x0, 0x0}, 0xc0001239b0)
    	[...]/avalanche/cmd/mtypes/main.go:127 +0xcbd
    main.main()
    	[...]/avalanche/cmd/mtypes/main.go:81 +0x5c5

Add a test to repro it.

Signed-off-by: Craig Ringer <[email protected]>

Author: ringerc

cmd/mtypes/main_test.go

@@ -1132,6 +1132,14 @@ func TestComputeAvalancheFlags(t *testing.T) {
 				"--metric-interval=0",
 			},
 		},
+		{
+			testName:               "noInput",
+			avalancheFlagsForTotal: 1000,
+			statistics:             map[dto.MetricType]stats{},
+			total:                  stats{},
+			expectedSum:            84,
+			expectedFlags:          []string{},
+		},
 	} {
 		t.Run(tc.testName, func(t *testing.T) {
 			avalancheFlags, adjustedSum := computeAvalancheFlags(tc.avalancheFlagsForTotal, tc.total, tc.statistics)