doc: add server side usage (#165)

Author: huang-julien

docs/content/1.getting-started/6.server-side-usage.md

@@ -0,0 +1,71 @@
+---
+title: Server side usage
+description: how to use nuxt-applicationinsights server side
+---
+
+# Getting started server side
+
+## In pure nitro context.
+
+In a pure nitro context (in the server dir), you can access to applicationinsights which uses [nitro-opentelemetry](https://github.com/huang-julien/nitro-opentelemetry).
+
+`nitro-opentelemetry` is a small nitro module to correctly hook events from nitro to opentelemetry and applicationinsights (since v3) makes use of it. 
+
+There is two ways of using applicationinsights in nitro. You can either access the singleton `Applicationinsights` by importing the default client (setup by [nitro-applicationinsights](https://nitro-applicationinsights.julien-huang.dev/)):
+
+
+```ts
+import Applicationinsights from "applicationinsights"
+
+export default defineTracedEventHandler((event) => {
+    // ...
+
+    Applicationinsights.trackEvent(/* ... */)
+
+    // ...
+})
+```
+
+Either modify the `Span` within the event.
+
+```ts
+export default defineTracedEventHandler((event) => {
+    // ...
+
+    event.otel.span.addAttribute(/* ... */)
+
+    // ...
+})
+```
+
+## Within a Nuxt app
+
+Within a Nuxt app, `nuxt-applicationinsights` will inject the `Applicationinsights` singleton to `useNuxtApp().$appInsights` server-side.
+
+Because `Applicationinsights` from `@microsoft/applicationinsights-web` has a very similar API to the server-side singleton of `applicationinsights`, `nuxt-applicationinsights` with create and inject `Applicationinsights` from `@microsoft/applicationinsights-web` to `useNuxtApp().$appInsights` client-side.
+
+```html
+<template>
+    <!---->
+</template>
+
+<script setup lang="ts">
+useNuxtApp().$appInsights.trackEvent(/* */)
+</script>
+```
+
+Since components are universal in Nuxt (run server and client side), if you're `opentelemetry`, you will need to put everything behind a `import.meta.server` flag.
+
+
+```html
+<template>
+    <!---->
+</template>
+
+<script setup lang="ts">
+
+if(import.meta.server) {
+    useNuxtApp().ssrContext!.event.otel.span.addAttribute(/* */)    
+}
+</script>
+```