<issue_start><issue_comment>Title: Problem assigning parent span to manually created spans
username_0: Please answer these questions before submitting a bug report.

### What version of OpenTelemetry are you using?

@opentelemetry/api@0.18.1
@opentelemetry/node@0.18.2
@opentelemetry/tracing@0.18.2

### What version of Node are you using?

nodejs-14.16.1-1.fc33.x86_64

### Please provide the code you used to setup the OpenTelemetry SDK

This is a very much reduced test case that uses the `NoopTracer`.

```
const opentelemetry = require('@opentelemetry/api');
const {NodeTracerProvider} = require('@opentelemetry/node');
const {SimpleSpanProcessor} = require('@opentelemetry/tracing');
const tracer = opentelemetry.trace.getTracer('basic');
```

### What did you do?

I want to manually create a span, and then manually create a child span.

```
const mainSpan = tracer.startSpan('test_context');
const childSpan = tracer.startSpan('test_context_2', {}, mainSpan.context());
```

However, this throws the below error
### What did you expect to see?

I want to initialize a new trace that has a parent trace. 

### What did you see instead?

Error thrown:

```
Uncaught TypeError: context.getValue is not a function
    at getSpan (/home/username_0/Lumii/lumii-design-portal/cloud-workers/previews/pdf_to_tif_sep/node_modules/@opentelemetry/api/build/src/context/context.js:39:20)
    at Object.getSpanContext (/home/username_0/Lumii/lumii-design-portal/cloud-workers/previews/pdf_to_tif_sep/node_modules/@opentelemetry/api/build/src/context/context.js:70:18)
    at NoopTracer.startSpan (/home/username_0/Lumii/lumii-design-portal/cloud-workers/previews/pdf_to_tif_sep/node_modules/@opentelemetry/api/build/src/trace/NoopTracer.js:34:54)
    at ProxyTracer.startSpan (/home/username_0/Lumii/lumii-design-portal/cloud-workers/previews/pdf_to_tif_sep/node_modules/@opentelemetry/api/build/src/trace/ProxyTracer.js:30:34)
```

### Additional context

I'm providing a link to the head of the git repo below but looking at the packages I pulled from npm the code here appears unchanged.

It seems like when initializing the second trace it should follow this codepath:

https://github.com/open-telemetry/opentelemetry-js/blob/d504f32a1866bb08aa4162a9ec2fa2c413043375/packages/opentelemetry-tracing/src/Tracer.ts#L70

The `getParent()` function should return the context of the parent span. Either I'm misunderstanding the code and docs or there is some internal mismatch around the context object. I'm not sure where `context.getValue()` is defined but I don't see it in the library version I have on disk from npm. 

I've also seen examples saying I ought to do something like
`tracer.startSpan('test_context_2', {parent: mainSpan})` but looking at the code in `Tracer.ts` I don't see a `paren`t property being used in `options`.

Thanks for your help!<issue_closed>
<issue_comment>username_0: Thank you! I even looked at that example and didn't get it. Sorry, and I really appreciate your help.