This PR implements the basis for a telemetry feature for the CLI in daemon mode.

Rationale:
Enhance the observability of the CLI internals at runtime, as we are proceeding internally to integrate the CLI as a gRPC server in all our services.

Module telemetry
Leverages https://github.com/segmentio/stats library to configure and expose a Prometheus endpoint when activated (in cli/daemon/daemon.go). It also provides helpers for transformation of commands input data in stats.Tags objects, using the installation id and secret generated by the inventory module.

The telemetry activation and endpoint config are available via CLI configuration.

Module repertory (feel free to ask for a name change 😄) inventory
This module leverages an additional Viper instance, in order to create a configuration repository that contains values that may create confusion in the user-facing arduino-cli.yaml file, polluting it.

This is the case for the installation values stored in the inventory.yaml file, that are used to generate server specific UUIDs that help the monitoring tools to perform statistics aggregation, when monitoring multiple CLI gRPC server instances.

The inventory file is created at the CLI first launch and it is stored in the directories.Data folder.

Instrumentation
Code instrumentation is done via standard stats apis for each command (only for the compile and board list in this PR):

1. Create the set of tags for the command

tags := map[string]string{
		"fqbn":            req.Fqbn,
		"sketchPath":      telemetry.SanitizeSketchPath(req.SketchPath),
		"showProperties":  strconv.FormatBool(req.ShowProperties),
		"preprocess":      strconv.FormatBool(req.Preprocess),
		"buildProperties": strings.Join(req.BuildProperties, ","),
...
                "success":          "true",

2. Modify the success tag according to each case

tags["success"] = "true"

3. Push the data to the stats engine

stats.Incr("compile", stats.M(tags)...)

Tests

• test_inventory_creation: python integ test
• test_inventory_prometheus_endpoint: python integ test

Notes

• Upgraded dependencies (Viper, pyinvoke as pyinvoke/invoke#660 was closed)
• Updated README.rst adding details about telemetry endpoint and config