Create constants for the configuration map

[cloudxxx8]

We need:

• Metadata
• Data
• Primary

They will be used in core-data, core-command, device services, and so on.

[lenny-goodell]

@cloudxxx8 , could you explain what configuration map in more detail?

[cloudxxx8]

For example, we have those in the configuration file:

[Clients]
  [Clients.Data]
  Protocol = 'http'
  Host = 'localhost'
  Port = 48080

  [Clients.Metadata]
  Protocol = 'http'
  Host = 'localhost'
  Port = 48081

Then, we need the constants for configuration map key instead of hardcode
https://github.com/edgexfoundry/edgex-go/blob/48f292ecce04635bad45b2dac08c505f02e60d07/internal/core/data/init.go#L61-L62

[lenny-goodell]

I agree we should have constants.
We have this old similar issue (In Icebox) on edgex-go that wants to use the service keys as the constants for the map keys.
edgexfoundry/edgex-go#2424
@jpwhitemn We should discuss this in Core WG meeting

[cloudxxx8]

this issue will be discussed in the next architects' meeting

[jpwhitemn]

Somewhat related to edgexfoundry/edgex-go#2424

Per the Monthly Architect's meeting of 1/26/21 - it was decided that
We will use service keys in configuration.toml files
Service keys are defined in https://github.com/edgexfoundry/go-mod-core-contracts/blob/master/clients/constants.go