Prometheus-Sample-App gauge metric value only goes up

[mustafain117]

Describe the bug

Prometheus gauge metric represents value that can go up and down. However, Gauge metric produced by this app currently only goes up.

What did you expect to see?

Expected to see gauge value go both up and down instead of being monotonically increasing.

Possible Solution

In function updateGauge() to c.Set(rand.Float64()). This should make gauge value go both ways - up and down.

Additional context
Related to open-telemetry/prometheus-interoperability-spec#57
cc @alolita @Aneurysm9

[PaurushGarg]

Merged as part of PR #11