Add optional Zero Threshold for Exponential Histograms

This is a field that exists and is used in Prometheus Exponential
Histograms and it would be nice to include it in OTel Exponential
Histograms as well.

You can see the reference implementation here:
https://github.com/prometheus/prometheus/blob/5937b4f5d4f658758f4aa6b9ab803f022e6e5497/model/histogram/float_histogram.go#L186-L197
https://github.com/prometheus/prometheus/blob/5937b4f5d4f658758f4aa6b9ab803f022e6e5497/model/histogram/float_histogram.go#L770-L790

Signed-off-by: Goutham Veeramachaneni <[email protected]>

specification/metrics/data-model.md

@@ -643,14 +643,22 @@ perfect subsetting.
 | 6                      | 1.68179        | 2**(6/8)                     |
 | 7                      | 1.83401        | 2**(7/8)                     |
 
-#### Zero Count
-
-The ExponentialHistogram contains a special `zero_count` field
-containing the count of values that are either exactly zero or within
-the region considered zero by the instrumentation at the tolerated
-level of precision.  This bucket stores values that cannot be
-expressed using the standard exponential formula as well as values
-that have been rounded to zero.
+#### Zero Count and Zero Threshold
+
+The ExponentialHistogram contains special `zero_count` bucket and an optional 
+`zero_threshold` fields where `zero_count` contains the count of values whose 
+absolute value is less than or equal to `zero_threshold`. The precise value 
+for the `zero_threshold` is arbitrary and not related to the scale.
+
+When `zero_threshold` is unset or `0`, this bucket stores values that cannot 
+be expressed using the standard exponential formula as well as values that 
+have been rounded to zero.
+
+Histograms with different `zero_threshold` can still be merged easily by 
+taking the largest `zero_threshold` of all involved Histograms and merge the 
+lower buckets of Histograms with a smaller `zero_threshold` into the common 
+wider zero bucket. In special cases, a wider zero bucket could be used to 
+limit the total number of populated buckets.
 
 #### Producer Expectations