Sampler is a mod by Player that generates .nps files.
You can find the forum thread over at IndustrialCraft Thread
For Minecraft 1.12
For Minecraft 1.11
For Minecraft 1.10
Then you can /sampler start and /sampler stop while it's lagging.
Then a simple /sampler export [name of exported nps file here] will write out the .nps file that can be read by the developers in VisualVM.
Important: where to run Sampler?
Is it a lag issue on the server, like tick lag? Run Sampler on the server, NOT on your client!
Is it a lag issue on the client, like FPS lag? Run Sampler on the client, NOT on your server!