chore: add plugin request issue template (open-telemetry#407)

* chore: add plugin request issue template

* Update .github/ISSUE_TEMPLATE/plugin_request.md

Co-Authored-By: dyladan <dyladan@users.noreply.github.com>

.github/ISSUE_TEMPLATE/plugin_request.md

@@ -0,0 +1,30 @@
+---
+name: Plugin request
+about: Create a report to add support for plugin
+labels: plugin-request
+---
+
+<!--
+**NB:** Before opening a plugin support request against this repo, consider whether the plugin should/could be implemented in the [other OpenTelemetry client libraries](https://github.com/open-telemetry/). If so, please [open an issue on opentelemetry-specification](https://github.com/open-telemetry/opentelemetry-specification/issues/new) first.
+
+You are welcome to try out the [plugin api](https://github.com/open-telemetry/opentelemetry-js/blob/master/doc/plugin-guide.md) to build your own plugin. If you do try out the plugin api, please let us know if you have any questions/feedback.
+-->
+
+**Is your plugin request related to a problem? Please describe.**
+<!--
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+-->
+
+**Is it applicable for Node or Browser or both**
+
+**Do you expect this plugin to be commonly used**
+Weekly Downloads:
+
+**What version of plugin are you interested in using**
+Versions:
+
+**Additional context**
+<!--
+Add any other context or screenshots about the plugin request here.
+-->
+- **Is there a reference you could point for the well-defined lifecycle methods**