Generate color key for flame graph

[alblue]

When a flame graph is generated with the -out flamegraph.html option, display a legend at the bottom that explains the colours used.

It might also be an idea to say whether or not the graph was generated from wall clock time, cpu time, allocation mode, or whether the display is for the total amount of time or number of samples.

That way, when the file is shared to another person, they immediately know what type of graph it is and what the colours mean.

[apangin]

Flame Graph title is generated now depending on the -e and --total options (unless overridden with --title).

E.g.

• when profiling -e wall => the title is "Wall clock profile"
• when profiling -e alloc --total => the title is "Allocation profile (bytes)"

[Sarkie]

@apangin
What about the colour legend?
Green is Java, yellow is C++, orange is kernel, and red is the remainder (native user-level, or kernel modules).

https://brendangregg.com/FlameGraphs/cpuflamegraphs.html

[apangin]

I'll add the legend in HTML later.