Span empty attributes are default mutable

[AndrewAXue]

Describe your environment
Python 3.7. Irrelevant.

Steps to reproduce

from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import (
    SimpleExportSpanProcessor,
)
from opentelemetry.sdk.trace.export import ConsoleSpanExporter

trace.set_tracer_provider(TracerProvider())

console_exporter = ConsoleSpanExporter()
trace.get_tracer_provider().add_span_processor(
    SimpleExportSpanProcessor(console_exporter)
)

tracer = trace.get_tracer(__name__)
with tracer.start_as_current_span('foo') as cur_span:
    cur_span.attributes['key'] = 'value'
with tracer.start_as_current_span('other'):
    pass

What is the expected behavior?

{
    "name": "foo",
    "context": {
        "trace_id": "0x500ef2fb2813de1abadf791877a0b05e",
        "span_id": "0x853082db85da172f",
        "trace_state": "{}"
    },
    "kind": "SpanKind.INTERNAL",
    "parent_id": null,
    "start_time": "2020-05-20T18:52:55.773473Z",
    "end_time": "2020-05-20T18:52:55.773584Z",
    "status": {
        "canonical_code": "OK"
    },
    "attributes": {
        "key": "value"
    },
    "events": [],
    "links": []
}
{
    "name": "other",
    "context": {
        "trace_id": "0xcaaed6c2f0d5e5d538401b7b212348cc",
        "span_id": "0x35ec0706aa1df654",
        "trace_state": "{}"
    },
    "kind": "SpanKind.INTERNAL",
    "parent_id": null,
    "start_time": "2020-05-20T18:52:55.773922Z",
    "end_time": "2020-05-20T18:52:55.773940Z",
    "status": {
        "canonical_code": "OK"
    },
    "attributes": {},
    "events": [],
    "links": []
}

What is the actual behavior?

{
    "name": "foo",
    "context": {
        "trace_id": "0x500ef2fb2813de1abadf791877a0b05e",
        "span_id": "0x853082db85da172f",
        "trace_state": "{}"
    },
    "kind": "SpanKind.INTERNAL",
    "parent_id": null,
    "start_time": "2020-05-20T18:52:55.773473Z",
    "end_time": "2020-05-20T18:52:55.773584Z",
    "status": {
        "canonical_code": "OK"
    },
    "attributes": {
        "key": "value"
    },
    "events": [],
    "links": []
}
{
    "name": "other",
    "context": {
        "trace_id": "0xcaaed6c2f0d5e5d538401b7b212348cc",
        "span_id": "0x35ec0706aa1df654",
        "trace_state": "{}"
    },
    "kind": "SpanKind.INTERNAL",
    "parent_id": null,
    "start_time": "2020-05-20T18:52:55.773922Z",
    "end_time": "2020-05-20T18:52:55.773940Z",
    "status": {
        "canonical_code": "OK"
    },
    "attributes": {
        "key": "value"
    },
    "events": [],
    "links": []
}

Additional context
Span._empty_attributes is a default mutable when the user (incorrectly) accesses a span's attributes directly. We can solve this by doing

        if not attributes:
            self.attributes = BoundedDict.from_map(MAX_NUM_ATTRIBUTES)
        else:
            self.attributes = BoundedDict.from_map(
                MAX_NUM_ATTRIBUTES, attributes
            )

i.e. deepcopying.

[AndrewAXue]

Addessed in #714