cmd/tracegen: Try to extract context from other params

[xswordsx]

Why was this pull request opened?

There are cases in which the traced interface does not accept a context.Context, but rather, uses the context from the passed down *http.Request. These cases can also be traced with a little change in the tracegen tool.

What does this Pull request do?

This PR addresses the above-mentioned case.

Anything worth noting?

The code does not check if the *Request is actually net/http or some other package (see TODO).

Can you provide a minimal example?

Example

Given the source code ifc.go in github.com/Bo0mer/gentools/ifc:

package ifc

import (
        "context"
        "net/http"
)

//go:generate tracegen . Spammer

// Spammer spams messages to given request.
type Spammer interface {
        Spam(*http.Request)
        SpamWithContext(context.Context, *http.Request)
        SpamContext(context.Context)
}

running the tool produces the following file:

// Code generated by tracegen. DO NOT EDIT.
package ifcmws

import (
        alias4 "context"
        alias1 "github.com/Bo0mer/gentools/ifc"
        alias2 "go.opencensus.io/trace"
        alias3 "net/http"
)

type tracingSpammer struct {
        next alias1.Spammer
}

// NewTracingSpammer creates new tracing middleware.
func NewTracingSpammer(next alias1.Spammer) alias1.Spammer {
        return &tracingSpammer{next}
}
func (m *tracingSpammer) Spam(arg1 *alias3.Request) {
        ctx := arg1.Context()
        ctx, _span := alias2.StartSpan(ctx, "github.com/Bo0mer/gentools/ifc.Spammer.Spam")
        defer _span.End()
        arg1 = arg1.WithContext(ctx)
        m.next.Spam(arg1)
}
func (m *tracingSpammer) SpamWithContext(arg1 alias4.Context, arg2 *alias3.Request) {
        arg1, _span := alias2.StartSpan(arg1, "github.com/Bo0mer/gentools/ifc.Spammer.SpamWithContext")
        defer _span.End()
        m.next.SpamWithContext(arg1, arg2)
}
func (m *tracingSpammer) SpamContext(arg1 alias4.Context) {
        arg1, _span := alias2.StartSpan(arg1, "github.com/Bo0mer/gentools/ifc.Spammer.SpamContext")
        defer _span.End()
        m.next.SpamContext(arg1)
}