zig fmt

update to zig 0.16.0

https://github.com/karlseguin/metrics.zig/issues/11

std.io -> std.Io

Remove deprecated writer usage in test_runner

Fix demo

fix deprecated things (#23096) and writergate (#24329) in 0.15 dev cycle

https://github.com/karlseguin/metrics.zig/issues/6

[Fix] example build issue

1. No need to call "try" to init gauge
2. Label type changed when passed via auxillary function via "anytype"

Fix concurrent create on counter and gauge vectors

Use `std.Build.path` in `build.zig`

This avoids 1 write to writer. Possible in the case of Gauge and Counters. Not
possible for any *Vec metric as well as plain Histograms.