fix(docs): replace emoji markup with an emoji glyph

Jekyll does not interpret markup like `:warning:` and as a result,
the Metrics page was showing the following heading:

    Stability: :warning:Experimental:warning:

In this commit, I am replacing `:warning:` with the actual emoji glyph,
i.e. `⚠️`. This fixes the rendered page.

Signed-off-by: Miroslav Bajtoš <mbajtoss@gmail.com>

extensions/metrics/README.md

@@ -3,7 +3,7 @@
 This module contains a component that reports metrics of Node.js, LoopBack
 framework, and your application to [Prometheus](https://prometheus.io/).
 
-## Stability: :warning:Experimental:warning:
+## Stability: ⚠️Experimental⚠️
 
 > Experimental packages provide early access to advanced or experimental
 > functionality to get community feedback. Such modules are published to npm