Add missing using instrumentation libraries pages: Erlang/Elixir (#4371)

Co-authored-by: Phillip Carter <[email protected]>

content/en/docs/languages/erlang/libraries.md

@@ -0,0 +1,55 @@
+---
+title: Using instrumentation libraries
+linkTitle: Libraries
+weight: 40
+cSpell:ignore: Ecto Hex
+---
+
+{{% docs/languages/libraries-intro "Erlang" %}}
+
+## Use instrumentation libraries
+
+If a library doesn't include OpenTelemetry support, you can use
+[instrumentation libraries](/docs/specs/otel/glossary/#instrumentation-library)
+to generate telemetry data for a library or framework.
+
+For example,
+[the instrumentation library for Ecto](https://github.com/open-telemetry/opentelemetry-erlang-contrib/tree/main/instrumentation/opentelemetry_ecto)
+automatically creates [spans](/docs/concepts/signals/traces/#spans) based on
+queries.
+
+## Setup
+
+Each instrumentation library is distributed as a Hex package. To install an
+instrumentation, add the dependency to your `mix.exs` file. For example:
+
+```elixir
+def deps do
+  [
+    {:opentelemetry_{package}, "~> 1.0"}
+  ]
+end
+```
+
+Where `{package}` is the name of the instrumentation.
+
+Note that some instrumentation packages might have prerequisites. Check the
+documentation of each instrumentation package for further instructions.
+
+## Available instrumentation libraries
+
+For a full list of instrumentation libraries, see the
+[list of Hex packages](https://hex.pm/packages?search=opentelemetry&sort=recent_downloads).
+
+You can also find more instrumentations available in the
+[registry](/ecosystem/registry/?language=erlang&component=instrumentation).
+
+## Next steps
+
+After you have set up instrumentation libraries, you might want to add your own
+[instrumentation](/docs/languages/erlang/instrumentation) to your code, to
+collect custom telemetry data.
+
+You might also want to configure an appropriate exporter to
+[export your telemetry data](/docs/languages/erlang/exporters) to one or more
+telemetry backends.

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