Handling missing keys

[mweidling]

In some cases, keys that represent relevant metrics might be missing.

One example is the cpu_time which misses in workflows where bashlib processors are used (unlike the Python based processors, they don't provide any information about the CPU time. To ensure – or at least to not skew – comparability – @MareenGeestmann and I discussed that it would be better to not provide any CPU time at all in these cases).

The current implementation of the Mongo API is configured to not display any keys which have a value of null (in accordance with @paulpestov ). The front end has to handle this fact, especially for the cases where a metric is affected.