"We found that though there are many backend products - an open source product with UI custom-built for observability, which integrates metrics & traces, was missing."
1. There are specific observabilty specific UI widgets like serviceMap, SLOs and error budgets, I don't know whether Grafana provides it now. Also, last I used Grafana, linking and moving from one dashboard to another is still a pain. You can get a better idea of how different observability UI can get from Grafana by looking into LightStep demo.
2. We can run aggregated on filtered traces. Eg, I can get 99th percentile response time of a tag say payment_channel. am afraid this can be extracted from traces by Grafana.
3. SigNoz is easily extendible by adding your stream processing application to slice n dice data in your own way