feat(Bigtable): add attempts headers on retry and exception logging

[bshaffer]

This PR adds two visibility features to the Bigtable client in v0.221.0 (which will apply to google/cloud-bigtable: 1.28.3)

1. The bigtable-attempt header for retry attempts

If a retryable exception is thrown, and the Bigtable client retries the call, all additional attempts will contain the header/metadata bigtable-attempt, which will start with "1" for the first retry and increment from there

2. The logger option for BigtableClient

By setting a PSR3-compatible logging client, the BigtableClient class will log retryable or non-retryable exceptions which occur during the readRows stream

// create any PSR3-compatible logging client
$logger = new Monolog\Logger();

// pass it in when creating the bigtable client
$bigtable = new Google\Cloud\Bigtable\BigtableClient(['logger' => $logger]);

// Or pass it in when calling Table::ReadRows
$table = $bigtable->table($instanceId, $tableId);
$rows = $table->readRows(['logger' => $logger]);

Note: In the case of an exception, the exception message (e.g. $e->getMessage()) will be logged using LoggerInterface::error.

[bshaffer]

Done in googleapis/google-cloud-php-bigtable#13