exploit support for multiple output.xml files

[fspegni]

see #63

[adiralashiva8]

Thanks for your time and effort for robotframework-metrics. Any specific reason for closing this pull request

I have query: Currently metrics report supports only single execution. Extra support we get through this changes is metrics process multiple records instead of rebot we do!

[fspegni]

The reason why I closed the PR is that it contained a bug. I fixed a bug and created a new PR, which also targets another pending issue.

About your query: you are right, the purpose of this change is not to rely on calling rebot before robotmetrics. In addition, my problem was that by calling rebot I was only able to produce a single report.html but not a single output.xml, thus even rebot (AFAIK) solved the problem of merging several output.xml onto one (I had to do it "by hand", which was sub-optimal).