Add Gauge benchmarks

[darielli]

OpenTelemetry-api 1.38.0 added support for gauge(including LongGauge and DoubleGauge), the pr added the Gaugebenchmark similar to the counter and histogram.

At the same time, the links in the readme are modified so that the links that originally pointed to other branches are redirected to the main branch.

I am not quite sure if the test results are reasonable because the difference is very large.