Add code and scheme generation

[pleshakov]

Proposed changes

Problem:

• Ensure the Attributes method of Exprotable interface are generated automatically for the telemetry data types.
• Ensure the avro scheme (.avdl) is generated automatically for the telemetry data types.

Solution:

• Add generator tool in cmd/generator that generates code (Attributes method) and the scheme.
• Generator can be used in //go:generate annotations in go source files.
• Generator has "generator" build tag so that it is not included into the telemetry library by default.
• Generator uses golang.org/x/tools/go/packages to parse source files.

Testing:

• Unit tests of parsing.
• Unit tests of code and scheme generation - ensuring non-zero output.
• Unit tests of generated code in cmd/generator/tests package
• Manual validation of the generated scheme (cmd/generator/tests/data.avdl) using Apache Avro IDL Scheme Support IntelliJ plugin.

CLOSES - #18

Note: if you'd like to experiment with code and scheme generation:

• update Data in cmd/generator/tests/data.go and/or Data2 in cmd/generator/tests/subtests/data2.go
• run make generate
• check the output of cmd/generator/tests/data_attributes_generated.go, cmd/generator/tests/data.avdl and cmd/generator/tests/subtests/data2_attributes_generated.go

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING
  guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[jjngx]

🚀

[pleshakov]

during meeting with @shaun-nx , we found two small bugs:

(1)
if generator generates attributes for a type declared in telemetry package github.com/nginxinc/telemetry-exporter/pkg/telemetry, the generated code will fail because it of the self-import

package telemetry
/*
This is a generated file. DO NOT EDIT.
*/

import (
	"go.opentelemetry.io/otel/attribute"

	"github.com/nginxinc/telemetry-exporter/pkg/telemetry" // self-import
)

(2)
When generated code in project like NGF in package telemetry (like github.com/nginxinc/nginx-gateway-fabric/internal/mode/static/telemetry), the generated code import this repo package telemetry "github.com/nginxinc/telemetry-exporter/pkg/telemetry, which creates problems. Solution - add alias to the exported telemetry package

package telemetry
/*
This is a generated file. DO NOT EDIT.
*/

import (
	"go.opentelemetry.io/otel/attribute"
	"github.com/nginxinc/telemetry-exporter/pkg/telemetry"
)

func (d *NGFResourceCounts2) Attributes() []attribute.KeyValue {
	var attrs []attribute.KeyValue

	attrs = append(attrs, attribute.Int64("Gateways", d.Gateways))
	

	return attrs
}

// problem - we want to use github.com/nginxinc/telemetry-exporter/pkg/telemetry.Exportable here, but end up using Exportable from this package.
var _ telemetry.Exportable = (*NGFResourceCounts2)(nil)

I will fix them shortly

[pleshakov]

(forced push to resolve go.mod conflicts with the main branch)

[ciarams87]

LGTM