Adding Prometheus metrics

This commit adds a '/metrics' endpoint in the API that can be exploited by Prometheus.

This endpoint returns:

- the number of connected sockets
- the number of users per room
- common NodeJS and system metrics

WARNING: this endpoint is public right now and should be protected

back/yarn.lock

@@ -266,6 +266,11 @@ better-assert@~1.0.0:
   dependencies:
     callsite "1.0.0"
 
+bintrees@1.0.1:
+  version "1.0.1"
+  resolved "https://registry.yarnpkg.com/bintrees/-/bintrees-1.0.1.tgz#0e655c9b9c2435eaab68bf4027226d2b55a34524"
+  integrity sha1-DmVcm5wkNeqraL9AJyJtK1WjRSQ=
+
 blob@0.0.5:
   version "0.0.5"
   resolved "https://registry.yarnpkg.com/blob/-/blob-0.0.5.tgz#d680eeef25f8cd91ad533f5b01eed48e64caf683"
@@ -1333,6 +1338,13 @@ progress@^2.0.0:
   version "2.0.3"
   resolved "https://registry.yarnpkg.com/progress/-/progress-2.0.3.tgz#7e8cf8d8f5b8f239c1bc68beb4eb78567d572ef8"
 
+prom-client@^12.0.0:
+  version "12.0.0"
+  resolved "https://registry.yarnpkg.com/prom-client/-/prom-client-12.0.0.tgz#9689379b19bd3f6ab88a9866124db9da3d76c6ed"
+  integrity sha512-JbzzHnw0VDwCvoqf8y1WDtq4wSBAbthMB1pcVI/0lzdqHGJI3KBJDXle70XK+c7Iv93Gihqo0a5LlOn+g8+DrQ==
+  dependencies:
+    tdigest "^0.1.1"
+
 proxy-addr@~2.0.5:
   version "2.0.6"
   resolved "https://registry.yarnpkg.com/proxy-addr/-/proxy-addr-2.0.6.tgz#fdc2336505447d3f2f2c638ed272caf614bbb2bf"
@@ -1683,6 +1695,13 @@ table@^5.2.3:
     slice-ansi "^2.1.0"
     string-width "^3.0.0"
 
+tdigest@^0.1.1:
+  version "0.1.1"
+  resolved "https://registry.yarnpkg.com/tdigest/-/tdigest-0.1.1.tgz#2e3cb2c39ea449e55d1e6cd91117accca4588021"
+  integrity sha1-Ljyyw56kSeVdHmzZEReszKRYgCE=
+  dependencies:
+    bintrees "1.0.1"
+
 text-table@^0.2.0:
   version "0.2.0"
   resolved "https://registry.yarnpkg.com/text-table/-/text-table-0.2.0.tgz#7f5ee823ae805207c00af2df4a84ec3fcfa570b4"