trace: add support for stacktraces

[odeke-em]

I was just going through useful signals to help distinguish a trace, I noticed in the OpenCensus Proto specs that we have a definition for StackTrace
https://github.com/census-instrumentation/opencensus-proto/blob/d3b7200dbd14743faa22c6642291d76030833cbd/src/opencensus/proto/trace/v1/trace.proto#L300-L350

Perhaps we should support for stacktraces as fields of a span and provide helpers.

Options available

• Go runtime package support
  The Go runtime package provides the ability to retrieve stack frames e.g.
  https://golang.org/pkg/runtime/#Frames

package main

import (
	"fmt"
	"runtime"
)

func main() {
	e := func() {
		pc := make([]uintptr, 10)
		n := runtime.Callers(0, pc)
		if n == 0 {
			return
		}
		pc = pc[:n]
		frames := runtime.CallersFrames(pc)

		for {
			frame, more := frames.Next()
			fmt.Printf("- more:%v | %s\n%#v\n", more, frame.Function, frame)
			if !more {
				break
			}
		}
	}

	d := func() { e() }
	c := func() { d() }
	b := func() { c() }
	a := func() { b() }
	a()
}

• Panic/Byteslice translators
  In the case of panics. if users invoke recover() or they can invoke
  runtime.Stack or runtime/debug.Stack and from that we can provide a translator that basically translator an error, string, byte slice into *StackFrames e.g. in package trace

func ParseStackFrames(source interface{}) (*StackFrames, error) 

where StackFrames is a translation of the proto definition

Exporter support

Stackdriver Trace supports exporting Stacktraces https://godoc.org/google.golang.org/genproto/googleapis/devtools/cloudtrace/v2#StackTrace
and I think other APM providers would be interested in following too.

[semistrict]

I think this needs to be specified in OpenCensus specs first.

[rakyll]

We deprecated stacktrace support from OpenCensus. AFAIK, they ended up being in the proto in the first place to comply with some legacy stuff we are not interested in supporting anymore. Please open an issue at the specs repo, https://github.com/census-instrumentation/opencensus-proto, if we need to reconsider them.