[LNT] Fix the graph page to show multiple metrics for the same test

Note: request.args is a dict and duplicate keys are ignored.
Currently the plot args have the following format plot.unused=m.t.f where unused is the test id.
If you select few metrics for the same test (for example compile_time and execution_time) on the Run page and click Graph, the URL will look like /db_default/v4/nts/graph?plot.958=5.958.2&plot.958=5.958.3
The second plot argument with the same key will be ignored. This patch fixes this issue.

OS Laboratory. Huawei Russian Research Institute. Saint-Petersburg

Reviewed By: cmatthews

Differential Revision: https://reviews.llvm.org/D118170