Make gauge metrics thread-safe

[ashwingopalsamy]

In pkg/gofr/metrics/register.go, the float64Gauge type maintains a map of observations without any synchronization mechanism.

For example: SetGauge modifies this map while HTTP handlers and other goroutines may concurrently read or update gauge values. As Go maps aren’t thread-safe for concurrent writes, simultaneous calls to SetGauge can lead to data races or undefined behaviors.