gstreamer/tracer

# Add a python api for tracer analyzers

The python framework will parse the tracer log and aggregate information.
the tool writer will subclass from the Analyzer class and override method like:

  'pad_push_buffer_pre'

There is one method for each hook. Each of those methods will receive the parse
log line. In addition the framework will offer some extra api to allow to e.g.
write:

  pad.name()             # pad name
  pad.parent().name()    # element name
  pad.peer().parent()    # peer element
  pad.parent().state()   # element state

If we don't have full loging, we'd like to print a warning once, but make this
non fatal if possible. E.g. if we don't have logging for
element_{add,remove}_pad, we might not be able to provide pad.parent().

A tool can replay the log multiple times. If it does, it won't work in
'streaming' mode though. Streaming mode can offer live stats.

## TODO
Do we want to provide classes like GstBin, GstElement, GstPad, ... to aggregate
info. We'd need to also provide a way to e.g. add a GstLogAnalyzer that knows
about data from the log tracer and populates the classes. We need to be able to
build a pipeline of analyzers, e.g. the analyzer calls GstLogAnalzer in its
catch-all handler and then processes some events individually.

Parse the tracer classes. Add helper that for numeric values extract them, and
aggregate min/max/avg. Consider other statistical information (std. deviation)
and provide a rolling average for live view.

Think of how analyzer classes can be combined:
- we'd like to build tools that import other analyzer classes and chain the
  processing.

## Examples
### Sequence chart generator (mscgen)

1.) Write file header

2.) collect element order
Replay the log and use pad_link_pre to collect pad->peer_pad relationship.
Build a sequence of element names and write to msc file.

3.) collect event processing
Replay the log and use pad_push_event_pre to output message lines to mscfile.

4.) write footer and run the tool.

## Latency stats

1.) collect per sink-latencies and for each sink per source latencies
Calculate min, max, avg. Consider streaming interface, where we update the stats
e.g. once a sec

2.) in non-streaming mode write final statistic

## cpu load stats

Like latency stats, for cpu load. Process cpu load + per thread cpu load.

## top

Combine various stats tools into one.

# Improve tracers
## log
* the log tracer logs args and results into misc categories
* issues
  * not easy/reliable to detect its output among other trace output
  * not easy to match pre/post lines
  * uses own do_log method, instead of gst_tracer_record_log
    * if we also log structures, we need to log the 'function' as the
      structure-name, also fields would be key=(type)val, instead of key=value
    * if we switch to gst_tracer_record_log, we'd need to register 27 formats :/