mtypes divide by zero crash if input has no summary or histogram metrics

[ringerc]

The mtypes utility crashes with a 0 divide error if the input lacks summary or histogram metrics and the -avalanche-flags-for-adjusted-series flag is passed, e.g.

$ ./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

This is true even if valid metrics are passed, so long as the metric set omits summary and/or histogram metrics.

[ringerc]

I've raised #137 to fix this