`ExponentialHistogram` is missing the `to_json` method

[AlecRosenbaum]

The ExponentialHistorgram data point type was somewhat recently added, and does not work with the ConsoleMetricExporter because it is missing a to_json method (which seems to be defined on the other data point types).

This is the stack trace I get locally:

Exception while exporting metrics 'ExponentialHistogram' object has no attribute 'to_json'
Traceback (most recent call last):
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/export/__init__.py", line 523, in _receive_metrics
    self._exporter.export(
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/export/__init__.py", line 163, in export
    self.out.write(self.formatter(metrics_data))
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/export/__init__.py", line 143, in <lambda>
    ] = lambda metrics_data: metrics_data.to_json()
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/point.py", line 252, in to_json
    "resource_metrics": [
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/point.py", line 253, in <listcomp>
    loads(resource_metrics.to_json(indent=indent))
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/point.py", line 233, in to_json
    "scope_metrics": [
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/point.py", line 234, in <listcomp>
    loads(scope_metrics.to_json(indent=indent))
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/point.py", line 211, in to_json
    "metrics": [
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/point.py", line 212, in <listcomp>
    loads(metric.to_json(indent=indent))
  File "/home/closeio/venv/lib/python3.10/site-packages/opentelemetry/sdk/metrics/_internal/point.py", line 193, in to_json
    "data": loads(self.data.to_json(indent=indent)),
AttributeError: 'ExponentialHistogram' object has no attribute 'to_json'

[rbtz-openai]

@ocelotl a friendly ping

(cc: @euroelessar @SaveTheRbtz)