filter_log_to_metrics: add new option discard_logs and code cleanup

By default, the filter don't drop the logs from the pipeline, there
are cases where only metrics are desired to reduce log volume.

This patch introduces a new option called discard_logs (default: false)
which when enabled will discard the logs once they are processed.

In addition, other changes were made:

- replacement of flb_malloc() with flb_calloc()
- context structure definition changes: e.g: do not malloc ints
- fix double free on cmetrics context destroy (histograms)
- general cleanups...

NOTE: I noticed that record accessor contexts are being created for fixed
patterns (e.g: $kubernetes['pod_name']), we will need to fix this since
the creation of those contexts are expensive, is times better to have them
in the context and use it on demand.

Signed-off-by: Eduardo Silva <[email protected]>