timechart legend in Azure Data Explorer

You can try customizing the series labels using the extend operator to create a new column, and then use that with project and render timechart.

requests
| where timestamp between (_startTime .. _endTime)
  and (url endswith "/emailtocase" or url endswith "/classify" or url endswith "/attachments")
| extend Endpoint = extract(@"([^/]+)$", 1, url)
| summarize Count = count() by bin(timestamp, 5m), Endpoint
| extend CustomLabel = Endpoint
| project timestamp, Count, CustomLabel
| render timechart with (series=CustomLabel)