move error tracking to context values, add multiple canaries to otel-cli status

[tobert]

#226 is asking for something I've wanted to do for a while. The first step in making this possible is to plumb context through the OTLP clients, so that there's a place to tunnel errors in and out so they can be displayed. This also presents a path to eliminate the final remaining global in otlpclient.Diag and replace it with context tunneling.

This PR also contains the addition of otlpclient/otlp_client_null.go that implements a non-recording client. While doing the context work I noticed I forgot to have CLI commands stop their clients. After adding that, otel-cli started panicing on nil pointers because the client was nil. NullClient fixes this.

Added otel-cli status --canary-count 1 --canary-interval 10ms functionality for sending multiple canaries.

[tobert]

@mterhar I got it this far for connection errors.

atobey@workbox ~/src/otel-cli $ go build && env -i PATH=/bin ./otel-cli status --endpoint localhost:44444

{
    "config": {
        "endpoint": "localhost:44444",
        "traces_endpoint": "",
        "protocol": "",
        "timeout": "1s",
        "otlp_headers": {},
        "insecure": false,
        "otlp_blocking": false,
        "tls_ca_cert": "",
        "tls_client_key": "",
        "tls_client_cert": "",
        "tls_no_verify": false,
        "service_name": "otel-cli",
        "span_name": "todo-generate-default-span-names",
        "span_kind": "client",
        "span_attributes": {},
        "span_status_code": "unset",
        "span_status_description": "",
        "force_span_id": "",
        "force_trace_id": "",
        "traceparent_carrier_file": "",
        "traceparent_ignore_env": false,
        "traceparent_print": false,
        "traceparent_print_export": false,
        "traceparent_required": false,
        "background_parent_poll_ms": 10,
        "background_socket_directory": "",
        "background_wait": false,
        "background_skip_parent_pid_check": false,
        "span_start_time": "now",
        "span_end_time": "now",
        "event_name": "todo-generate-default-event-names",
        "event_time": "now",
        "config_file": "",
        "verbose": false,
        "fail": false
    },
    "span_data": {
        "is_sampled": "true",
        "span_id": "f293c03186f0b7ae",
        "trace_id": "32cc67787a086322c6b1a90b51bda1e6"
    },
    "env": {
        "PATH": "/bin"
    },
    "diagnostics": {
        "cli_args": [
            "status",
            "--endpoint",
            "localhost:44444"
        ],
        "is_recording": true,
        "config_file_loaded": false,
        "number_of_args": 3,
        "detected_localhost": true,
        "insecure_skip_verify": false,
        "parsed_timeout_ms": 1000,
        "endpoint": "grpc://localhost:44444",
        "endpoint_source": "general",
        "error": "rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing: dial tcp 127.0.0.1:44444: connect: connection refused\"",
        "exec_exit_code": 0,
        "retries": 4
    },
    "errors": [
        {
            "Timestamp": "2023-06-23T15:21:09.476353318-04:00",
            "Error": "rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing: dial tcp 127.0.0.1:44444: connect: connection refused\""
        },
        {
            "Timestamp": "2023-06-23T15:21:09.876525485-04:00",
            "Error": "rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing: dial tcp 127.0.0.1:44444: connect: connection refused\""
        },
        {
            "Timestamp": "2023-06-23T15:21:09.876534878-04:00",
            "Error": "rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing: dial tcp 127.0.0.1:44444: connect: connection refused\""
        }
    ]
}