Implement telegraf collecting stats on itself

CHANGELOG.md

README.md

agent/accumulator.go

agent/accumulator_test.go

agent/agent.go

internal/buffer/buffer.go

internal/buffer/buffer_test.go

internal/config/config.go

internal/models/makemetric.go

internal/models/running_aggregator.go

internal/models/running_input.go

internal/models/running_input_test.go

internal/models/running_output.go

internal/models/running_output_test.go

plugins/inputs/all/all.go

plugins/inputs/http_listener/http_listener.go

plugins/inputs/internal/README.md

plugins/inputs/internal/internal.go

plugins/inputs/internal/internal_test.go

plugins/inputs/tcp_listener/tcp_listener.go