Add a basic http OpAMP client

[xrmx]

Description

This introduces a basic OpAMP http client for handling remote configuration. The client implements a bunch of capabilities (ReportsStatus, ReportsHeartbeat, AcceptsRemoteConfig, ReportsRemoteConfig) that are enough to get a remote config from an opamp server, parse it, apply it and ack it. Since OTel / OpAMP do not standardize APIs, config options or environment variables distro are required to provide code doing so.
OTel Python distros would need to provide their own message handler callback that implements the actual change of whatever configuration their backends sends.

In practice distro would need to do something like the following:

from opentelemetry._opamp.agent import OpAMPAgent
from opentelemetry._opamp.client import OpAMPClient
from opentelemetry._opamp.proto import opamp_pb2 as opamp_pb2


def opamp_handler(agent: OpAMPAgent, client: OpAMPClient, message: opamp_pb2.ServerToAgent):
    for config_filename, config in messages._decode_remote_config(message.remote_config):
        print("do something")


class MyOpenTelemetryConfigurator(_OTelSDKConfigurator):
    def _configure(self, **kwargs):
        super()._configure(**kwargs)

        enable_opamp = False
        endpoint = os.environ.get(OTEL_OPAMP_ENDPOINT)
        if endpoint:
            # this is not great but we don't have the calculated resource attributes around
            # see https://github.com/open-telemetry/opentelemetry-python/pull/4646 for creating
            # an entry point distros can implement
            resource = OTELResourceDetector().detect()
            agent_identifying_attributes = {
                "service.name": resource.attributes.get("service.name"),
            }
            opamp_client = OpAMPClient(
                endpoint=endpoint,
                agent_identifying_attributes=agent_identifying_attributes,
            )
            opamp_agent = OpAMPAgent(
                interval=30,
                message_handler=opamp_handler,
                client=opamp_client,
            )
            opamp_agent.start()

The module is called _opamp because it's a bit early to standardize on an api. The code is divided roughly in:

• agent: handles threads and queues for sending messages to the server, supports sending heartbeat messages at a fixed interval
• client: expose the api to build and send the OpAMP messages
• messages: wrappers to simplify protobuf serialization
• transports: http backends, currently there is only one using requests (to match the exporters)

OpAMP reference: https://opentelemetry.io/docs/specs/opamp/.

This is tested against https://github.com/elastic/opentelemetry-collector-components/ that is using the opamp-go implementation.

TODO:

• write some docs with usage examples

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated