Fix trace name resolution for spans with remote parents

[tmthrgd]

When encountering a trace whose root span contains a reference to a remote parent span in a different trace, the UI would fail to determine the root span and instead display the fallback <trace-without-root-span> trace name.

This change makes it so that the first span that has no parents in the current trace is correctly considered the root span. Previously only a span with no references/parents was considered
the root span.

To simplify the logic and ensure consistency, transformTraceData was changed to use the fixed getTraceName.

Traces with remote parents are commonly used when handling HTTP or RPC requests from untrusted clients. See the opencensus-specs util/HandleUntrustedRequests.md for an example of this approach.

I understand that I should add a test for this, but I'm unsure where would be appropriate. If someone can point me to an appropriate file, I'll add that to this PR.

An example of a trace that contains a remote parent.

{
    "data": [
        {
            "traceID": "46412397683b6cee4f02fa1ac7e4d0db",
            "spans": [
                {
                    "traceID": "46412397683b6cee4f02fa1ac7e4d0db",
                    "spanID": "7ef1d2e931ada231",
                    "operationName": "operation",
                    "references": [
                        {
                            "refType": "CHILD_OF",
                            "traceID": "79b24a4a9fb5d4e46895ab64f55dfc90",
                            "spanID": "9111732227172a77"
                        }
                    ],
                    "startTime": 1257894000000000,
                    "duration": 1000,
                    "tags": [],
                    "logs": [],
                    "processID": "p1",
                    "warnings": null
                }
            ],
            "processes": {
                "p1": {
                    "serviceName": "service",
                    "tags": []
                }
            },
            "warnings": null
        }
    ],
    "total": 0,
    "limit": 0,
    "offset": 0,
    "errors": null
}

Updates #117
Updates #134

[everett980]

@tmthrgd Thanks for submitting this PR.
The best place to test /src/model/trace-viewer would be a new file: /src/model/trace-viewer.test.js
/src/model/transform-trace-data unfortunately does not have a dedicated test file yet, and I think that would be beyond the scope of this PR.

Additionally, if you could convert trace-viewer to typescript (and update the suffix to .tsx) that would be great. I believe the only change necessary would be:

import { SpanData } from '../types/trace';

// eslint-disable-next-line import/prefer-default-export
export function getTraceName(spans: SpanData[]): string {

[yurishkuro]

This doesn't seem like an equivalent or a superset of the previous logic.

I would rather fix this like this:

• find all spans that either have no parent reference, or a parent reference that is not found in the trace
• if only one such span is found, it's the root (i.e. source of "trace name")
• if more than one such span is found, then
  • if exactly one of them has no parent refs, it's the root
  • otherwise give up (or, could still do something like pick the one with the earliest start time)

Clearly, we need a set of unit tests for this logic.

[tmthrgd]

Looks like this was superseded and fixed by #541. 🎉