doc: add gauge apis (#1149)
doc: add gauge apis (#1149)

Add doc for `Gauge` APIs introduced in https://github.com/lyft/envoy-mobile/pull/1117.

Risk Level: Low
Signed-off-by: Jingwei Hao <87eb6e61b06a9638bebd8a6383dcf233f789f5fe@gmail.com>
envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile